UNIVERSITY MANUALS EDITED BY PROFESSOR KNIGHT THE PHYSIOLOGY OF THE SENSES THE Physiology of the Senses BY JOHN GRAY M'KENDRICK, M.D., LL.D., F.R.SS.L. AND E. PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF GLASGOW AND WILLIAM SNODGRASS, M.A., M.B., C.M. MU1RHEAD DEMONSTRATOR OF PHYSIOLOGY IN THE UNIVERSITY OF GLASGOW FOURTH IMPRESSION (FIFTH THOUSAND) WITH 127 ILLUSTRATIONS LONDON JOHN MURRAY, ALBEMARLE STREET 1902 «i • 10. A II rights reserved » GULIELMO • TENNANT • GAIRDNER MEDICINE • APUD • UNIVERSITATEM • GLASGUENSEM • P ROFESSOR1 HUNC • LIBELLUM COLLEGA • COLLEGE DISCIPULUS • MAGISTRO AMANTISSIMO • UTERQUE • ANIMO DEDICAVERUNT AUTHORS' PREFACE IT is the aim of this book to give a succinct account of the functions of the organs of sense as these are found in man and the higher animals. The Authors have refrained from discussing with fulness of detail either the comparative physiology of the senses or the numerous interesting questions of a psychological character that inevitably arise in connection with the study of the mechanism of sensory perceptions. Each of these aspects of the subject would require a volume for itself. On the other hand, a perusal of this volume, which has been written so as to be readily understood even by those who have not made physiology a special subject of study, will be a suitable preparation for entering upon the more recondite questions that underlie physiological psychology. The Authors have endeavoured to treat the physiology of the senses as fully as space would allow, and have also suggested comparatively simple experiments by which any one interested in the subject may test some of the statements for himself. They would also direct attention to the last chapter, in which an attempt is made to elucidate the nature of the physiological basis of sensa,- viii Physiology of the Senses tion, in the hope that it may be found to be a contribution to speculative thought on this problem. While every page has been subjected to the careful consideration and revision of both Authors, it may be mentioned that the Introduction and the sections on Sight and Hearing have been mainly written by Dr. Snodgrass. J. G. M. W. S. UNIVERSITY OF GLASGOW, March 1893. CONTENTS GENERAL INTRODUCTION SENSORY MECHANISM PAGE Terminal organs of sensory nerves ..... 2 Nerve matter and nerves . . . . . . . 3 Nature of nerve current . . . . . . 5 Origin of nervous system ....... 7 Structure of nerves and nerve cells ..... 10 PATHS OF NERVOUS IMPULSES The spinal cord ......... 12 The medulla oblongata . . . . . . 19 The cerebellum . , . . . . . . .19 The pons ... ....... 21 The cerebrum ......... 22 SENSORY CENTRES IN THE CORTEX OF THE BRAIN The centre for vision ........ 30 The centre for hearing ........ 32 The centres for taste and smell ...... 34 The centre for touch ........ 34 The muscular sense ........ 36 Physiology of the Senses RELATION OF STIMULUS AND SENSATION PAGB Quality of sensation 36 Quantity of sensation . . . . . . . -37 Sensations and perceptions . 39 THE SENSE OF TOUCH Structure of the skin . . . . . . . -41 (1) The true skin .. . . . . . 42 (2) The epidermis ;. . . . . .42 Structure of tactile organs . . . . . . . 45 (1) Free nerve-endings . . . . . . ' . 45 (2) Nerve-endings in corpuscles . . . . -45 (3) Nerve-endings in connection with tactile hairs . . 50 Nature of the tactile mechanism ....»•-. . . 52 Sensitiveness of the skin . . .„..'. . . 54 Sense of locality . . . . . . . . 56 Absolute sensitiveness . . . -. \ . ' . . . 56 Fusion of tactile impressions . . . . . . 58 After-tactile impressions . .•_, ... . 58 Information from tactile impressions* . . . . . 59 Theories as to touch . . . . . . . .62 Sensations of temperature . . » . . .64 Sensation of pain . . . . . - . . . -67 The muscular sense . . 68 THE SENSE OF TASTE The organs of taste ........ 70 Minute structure of the gustatory organ . . . -71 Physical causes of taste . . . . . • • 73 Physiological conditions of taste . . . . . -74 Differentiation of tastes ....... 76 General sensibility of the tongue ..... 78 Subjective tastes ........ 78 Nerves of the tongue ....... 78 Contents xi THE SENSE OF SMELL PACK The organs of smell ........ 80 Physiological anatomy of the nose . . . . .81 Physical causes of smell ....... 86 Chemical nature of odorous substances . . 87 Flowers and odours ....... 89 Odour and heat absorption ...... 89 Odours and ozone ....... 90 Odours and surface tension ...... 90 Special physiology of smell . . . . . . 91 Mode of excitation of the olfactory nerves .... 93 THE SENSE OF SIGHT I. — STRUCTURE OF THE EYE Coats of the eyeball 97 Contents of the eyeball . . . . '. 105 The optic nerve . . . . . . . . . 109 Movements of the pupil . . . . . . .in II. — PHYSIOLOGY OF VISION i. — Laws of Dioptrics The physical nature of light . . . . . . 115 Reflection and refraction . . . . . . .116 Action of lenses . . . . . . . . .119 Formation of images by biconvex lenses . . . . 1 20 Spherical aberration . . . . . . . .122 Chromatic aberration . . . . . . . .124 Optical properties of a system of lenses . . . . .125 2. — The Dioptric System of the Eye Focal points . . . . . . . . .128 Principal points . . . . . . . . .129 Nodal points ......... 129 xii Physiology of the Senses 3. — Anomalies in the Eye as an Optical Instrument PAGE 1. Divergence of optic from visual axis .... 131 2. Divergence of line of regard from line of vision . . 131 3. Chromatic aberration . . . . . . -131 4. Spherical aberration . . . . . . .132 5. Astigmatism . . . . . . . . .132 4. — Adjustment of the Eye for different Distances The near point of vision . ... . . . 137 Irradiation . . . . . . . . . .140 Entoptic phenomena . . . . . . . . 141 Examination of the interior of the eye . . . . . 143 The visual angle ... ...... 145 The size of the retinal image . . . . . .148 The blind spot . . . . . . . . . 149 Action of light on the retina . . . . . . -150 Amount of light required to excite the retina . -. .- . 152 Persistence of retinal impressions . . . . . .152 5. — Sensation of Colour Complementary colours . . . . . . .158 Colour as dependent on the retina . . . . .158 Colour blindness . . . • .'' • . . . . 159 Coloured after-images . . „ . . . .161 Theories of colour vision . . . . . .161 6. — Binocular Vision Movements of the eye !7O The ocular muscles . . . . . . . .172 How an object is seen as one with two eyes . . . • J75 Perception of solidity . . . . . . . .180 The stereoscope ......... 181 The telestereoscope . . . . . . .184 Contents xiii PAGE Estimation of distance ........ 187 Estimation of size . . . . . . , .190 Illusions of vision ........ 192 SOUND AND HEARING The external ear ......... 200 External meatus . . . . ' . . . . 202 The middle ear . . . . . . . . . 204 The Eustachian tube . . ... . . . . 207 The chain of bones . . . . . . . . . 209 Movements of the bones . . . . . . .211 Response of the tympanic membrane to sound waves . . 214 Transmission of vibration by the auditory ossicles . . . 218 THE INTERNAL EAR The osseous labyrinth ........ 223 The auditory nerve . . . ... . . . 225 The membranous labyrinth ....... 225 The cochlea 228 The cochlear canal ..... ... 230 The organ of Corti . . . . . . . 231 The inner hair-cells . . . . . . . . 233 The outer hair-cells ........ 235 Innervation of the cochlea ....... 237 AUDITORY SENSATIONS Physiological characters of sounds ..... 240 (1) Pitch 242 (2) Intensity or loudness ... . 246 (3) Quality, timbre, or klang ...... 247 Resonators .......... 251 Analysis of compound tones by resonators .... 252 Noise ........... 262 General mode of action of the ear ...... 263 Analytic power of the ear ....... 269 xiv Physiology of the Senses THE PSYCHICAL ELEMENTS IN AUDITORY SENSATIONS PAGE Externality of sound . . . . . . . • 277 Direction of sound . . . . ' . . . . 280 Distance of the source of sound .;.... 281 Memory of sound ......... 282 Mental receptivity for sound ...... 283 Binaural audition ... ...... 283 THE PHYSIOLOGICAL CONDITIONS OF SENSATION APPENDIX I The action of light on the retina ...... 299 APPENDIX II Derivations of scientific terms . . . . •.'••„ 302 INDEX in LIST OF ILLUSTRATIONS FIG. PAGE 1. Cells of various Forms ...... 7 2. Neuro-epithelial Cell 8 3. Section of Spinal Cord ...... 10 4. Nerve Fibres ........ II 5. Multipolar Nerve Cells 12 6. Pyramidal Nerve Cells 12 7. Section of Spinal Cord ...... 16 8. Base of the Brain 21 9. Diagram of Encephalon ...... 23 10. Diagram of Side of Brain . . . . . 25 11. Median Aspect of Cerebral Hemisphere ... 27 12. Section of Skin ....... 41 13. Grandry's Corpuscles ...... 46 14. Wagner's Corpuscle ....... 47 15. Krause's End-bulb ........ 48 1 6. Large End-bulb 48 17. Nerves with Pacinian Corpuscles .... 49 1 8. Pacinian Corpuscle ....... 50 xvi Physiology of the Senses FIG. 19. Weber's Compasses 54 20. Sieveking's ^Esthesiometer ..... 55 21. Aristotle's Experiment ...... 61 22. Goldscheider's Cold and Hot Spots .... 64 23. Papilla Foliata . 71 24. Taste bud 73 25. Section of Nasal Cavities ...... 80 26. Outer side of Nares . ... • - • • • 82 27. Olfactory Region of Rabbit .. . . . . 84 28. Olfactory Cells . . . .... . 85 29. Section of Eyelid ..... V . 96 30. Diagram of Eyeball . . . - . . • 98 31. Section of Cornea . . . V . . . 99 32. Section of Conjunctiva . . \. . -. . 100 33. Ciliary Region of Eye . . . . . . 101 34. Vessels of Choroid and Iris 102 35. Retina . . 103 36. Retina . . .... . . . . 104 37. Rods and Cones . ,'/ 105 38. Ends of Rods and Cones . . . . . . 106 39. Pigment Cells . . = 106 40. Fibres of Lens 107 41. Diagram of Lens ...... 108 42. Structure of Lens . . . . . •. 109 43. Nerve Fibres in Retina . . . . . 109 44. Optic Decussation . . . . . . . no 45. Reflection of Light 117 List of Illustrations xvii PAGE 46. Refraction of Light . . . . . . . 118 47. Prism 119 48. Lenses ......... 119 49. Biconvex Lens ........ 120 50. Conjugate Foci . . . . . . . 120 51. Virtual Focus ........ 120 52. Formation of Image ....... 121 53. Effect of Absence of Lens from the Eye . . . 122 54. Use of Lens in Formation of Image in the Eye . . 122 55. Spherical Aberration . . . . . . 123 56. Chromatic Aberration . . . . . . 124 57. Achromatic Lens . . . . . . . 125 58. Course of a Ray through a Dioptric System . . 126 59. Image of a Point ....... 127 60. Schematic Eye . . . . . . . . 130 61. Astigmatism ........ 132 62. Cylindrical Lens for Astigmatism . . . . 133 63. Adjustment of Eye for Distance . . . . 134 64. Mechanism of Accommodation . . . . . 135 65. Reflected Images in Eye . . . . . . 136 66. Phakoscope . . . . . . . . 136 67. Schemer's Experiment . . . . . . 137 68. Different Forms of Eye . . . . . . 139 69. Irradiation . . . . . . . . 140 70. Formation of Purkinje's Figures .... 142 71. Principle of the Ophthalmoscope . . . . 144 72. The Visual Angle ....... 146 xviii Physiology of the Senses PAGE 73. Small Retinal Images . . . . . . 147 74. The Blind Spot 150 75. Fusion of Retinal Impressions 153 76. Lambert's Method of studying Combinations of Colours 156 77. Diagram to illustrate the Young-Helmholtz Theory of Colour Vision 162 78. Diagram to illustrate Hering's Theory of Colour Vision . . ... . . . . 166 79. The Visual Field " . 171 80. Diagram of Ocular Muscles 172 81. Section through the Orbit and its Contents . . 174 82. Binocular Visual Field . . . . . . 176 83. The Horopter 177 84. Formation of Homonomous Images . . . . 178 85. Formation of Heteronomous Images . . . . 179 86. Truncated Cone seen from above . . . . 181 87. Wheatstone's Stereoscope . . . . . . 182 88. Brewster's Stereoscope . . . . . . 183 89. Telestereoscope . . . . . . . 185 90. Causation of Luminosity . . . . . . 187 91. Estimation of Distance . . . . . . 189 92. Estimation of Space . . . . . . . 189 93. Visual Angle in Estimation of Size .... 190 94. Estimation of Size 191 95. Error of Judgment in Estimation of Size . . . 192 96. Zollner's Lines . . f 192 List of Illustrations xix FIG. PAGE 97. Illusion of Vision . . . . . . . 193 98. Perception of Solidity . . . . . . 195 99. Auditory Vesicle of Phialidium . . . . . 199 100. Right Auricle . . . . . . . . 201 101. Diagram of the Ear ....... 203 102. Left Tympanic Membrane ..... 205 103. Horizontal Section through Ear .... 206 104. Incus and Malleus in Tympanum . . . 207 105. Malleus . . . • . . . . . 209 106. Incus ......... 210 107. Stapes ......... 211 108. Leverage Action of Malleus and Incus . . . 213 109. Vibrating Strings . . . . . . . 215 no. Wave-forms ........ 220 in. Osseous Labyrinth ....... 223 112. Formation of Semicircular Canals . . . . 224 113. Membranous Labyrinth ...... 225 1 14. Section of Macula Acustica ..... 226 115. Epithelium of Macula ...... 227 116. Otoconia or Otoliths 227 117. Osseous Cochlea ....... 228 118. Section through Coil of Cochlea .... 229 119. Section through Cochlear Duct . . . . . 231 120. RodsofCorti 232 121. Surface View of Corti's Organ ..... 233 122. Section of Corti's Organ ..... . 234 123. Diagram of Change in Breadth of the Basilar Membrane 239 xx Physiology of the Senses PAGE 124. Double Syren ........ 243 125. Pendular Vibrational Curves ..... 249 126. Resonator 251 127. Konig's Apparatus for studying Vibration of Air in Organ Pipes ....... 254 GENERAL INTRODUCTION THE senses are called into play when the condition of the body has been affected to a certain degree by external or internal agencies. A flash of light, a piercing sound, a gentle touch, may so act upon the bodily organism as ta be followed by a sensation or mental state, by the conscious- ness of an alteration that has taken place in the body or in its environment. Sensitiveness is a property of all animals, and possibly of not a few plants. Some animals, indeed, are so low in the scale of organisation as to have no special parts set aside for the reception of sensory impressions, but every part of their body seems alike fitted to recognise varia- tions in its surroundings. As soon, however, as we pass to the higher grades of animal life we find certain parts or organs of sense whose duty is to keep the body in touch with its surroundings, and a nervous system which receives impressions and ensures the co-operation of all the individual elements of the body one with another. In order that sensations may be felt, we are provided with a central nervous system, or sensorium, from which nerve fibres pass outwards to all parts of the body, and at the ends of the nerve fibres certain structures or terminal organs may be found, which are so formed as to be capable of responding to some special variety of impression. Thus the terminal organ of the nerve of vision is insensitive to B 2 Physiology of the Senses the vibrations which, by acting upon the ear, originate changes leading to the sensation of sound. But, as will be shown in greater detail hereafter, this receptivity is largely conditioned by the special function of each sensory nerve centre. For the sensorium does not act as a whole, but is differentiated so that one part is devoted to one sense, another to another; and when the nerves which lead to these nerve centres have been stimulated, it matters not what the nature of the stimulus to the nerve has been, the sensation experienced is always for each centre of one and the same kind. Thus the optical centre always gives rise to the sensation of seeing something, the auditory centre to that of hearing, the olfactory centre to sensations of smell, the gustatory centre to those of taste, and the tactile centre to touch. But, over and above these special forms of sensation, there are many vague or general sensations, such as those of heat or cold, of pain or fatigue, of pressure, resistance, and the like, which may seem to be felt in almost every part of the body ; and although each of these has in all probability its special nerve centre, yet no special terminal organ seems to be necessary. Special terminal organs, then, are developed for the senses of sight, hearing, smell, taste, and touch : their structure will be described when we consider these senses separately. While we may readily distinguish these organs from one another by examination, either with the naked eye or the microscope, it is quite otherwise when we come to studv the nerve fibres or nerve centres. So far as we. can as yet determine, the nerve fibres which transmit the various sensory impressions are all of exactly the same composition and structure ; and though in recent times it has been found possible to localise with considerable accuracy the centres which are related to special sensations, still it has not been possible to fix upon the exact microscopical General Introduction 3 elements concerned ; in other words, physiologists cannot define the particular structure which alone is concerned in a given special sensation. We have no means of observing directly the minute molecular changes which go on in nervous substance ; we know only that this substance is very complex, and that during life it undergoes continual change, and is being constantly built up and broken down ; but neither the microscope nor chemical analysis has hitherto enabled us to determine why one centre should respond to one form of physical change, and another to another ; or why, when one part is stimulated, we have one kind of sensation, and when another part acts we have a different kind. A brief consideration of the composition and structure of nerve fibres and of nerve centres will enable us, however, to understand better the mechanism required for the trans- mission and recognition of a sensory impression. Nerve matter consists mainly of a variety of the sub- stance called protoplasm, which is composed of a network of exceedingly fine fibres, the meshes of which are filled up with a fluid or semi-fluid substance. The exact chemical nature of protoplasm cannot be stated, for, in the first place, it is constantly varying during life by taking up nutrient matter of different kinds, and by throwing off certain waste substances, the product of vital action ; and in the second place, whenever we try to subject it to chemical analysis, it dies and is broken up into simpler chemical compounds. The most important chemical elements found in protoplasm are Carbon, Oxygen, Hydrogen, Nitrogen, Sulphur, and Phosphorus, and they are combined in such quantities and proportions as to form molecules of a highly complex nature. Now the more complex a chemical compound is, the more unstable it is ; or, in other words, the more easily may it be broken up, and resolved into simpler substances ; and hence we have in nervous tissues, which are largely com- 4 Physiology of the Senses posed of protoplasm, a material which may be very readily changed when acted upon by external forces. That a change does take place in nerve matter, when in action, has been inferred, although we cannot tell what the exact chemical constitution of nervous matter is, nor how it is changed. We know that for the efficient working of the nervous system there must be a full and unrestricted blood supply, bringing fresh nutrient matter to make up for waste, and oxygen, to promote chemical changes. The blood, again, must be free from impurities, or nerve action will be disordered. Surrounding nerve fibres we find a system of fine spaces or channels into which waste products of nerve action are poured, so as to secure their ready removal. When a nerve is acting we can also detect electrical changes corresponding in all probability to chemical trans- formations of nerve substance, but it must be admitted that no proof has yet been given of chemical changes in a nerve. Nerves. — When a nerve has been kept in action for some time it apparently becomes fatigued — that is to say, the irritation of the nerve ceases to be followed by the usual result. Thus, if we irritate a nerve passing to a muscle, the muscle at first responds by contracting, but by and by the stimulations of the nerve fail to call forth contraction. We then say the nerve is fatigued, and we may suppose that its vital activity is diminished from lack of time to build up its wasted substance, or from the ac- cumulation of waste products which prevent free action. Of late, however, physiologists are gradually coming to the opinion that there is no direct evidence of fatigue in the nerve itself, and that the phenomena on which fatigue depends really occur in the apparatus or structure at the end of the nerve. From this point of view, nerve fibres may be regarded as not subject to much tear and wear, and they may act more like metallic conductors conveying General Introduction 5 currents of electricity, in which the current does not produce what are usually called chemical phenomena. Nature of Nerve Current. — Structures known as nerve cells maintain the nutrition of nerve fibres. If a fibre is cut off from the cell with which it is connected it soon degenerates, and can no longer transmit a nerve current. But in a healthy nerve fibre a change known as a " nerve current " passes along it in both directions of its length from the point of stimulation. This change may be of a chemical kind, although, as already pointed out, there is no proof of this, and certain facts point the other way. We may imagine, on the chemical hypothesis, the fine nerve fibre as containing very complex and unstable molecules, which are readily broken up when acted upon by some external force. And just as when a match is set to one end of a train of gunpowder, the chemical change in the first granules of powder liberates energy, which gives rise to action in adjoining granules with disintegration of their substance and the formation of simpler compounds, so in nerve the change in one part or molecule may give rise to changes in adjoining molecules, and a so-called current will pass along the fibre. The fact that one current may follow another with great rapidity shows that the nerve substance is altered only in part and is quickly regenerated ; but, on the other hand, the too frequent or prolonged application of a stimulus is followed by diminished power of conductivity by a nerve, or of receptivity in the nerve centres. It was at one time sup- posed that the nerve current might be a purely electrical change, and that it travelled with the lightning velocity of the electric current. And no doubt in our ordinary experience this seems to be the case. If the skin be touched with a red-hot iron wire, we seem at the same instant to feel the heat and pain. But by means of ap- paratus for registering minute intervals of time, and by 6 Physiology of the Senses stimulating a nerve in different parts of its length, we have ascertained that the rate of the nerve current is much slower than it would be were it purely electric ; and while there may be electric disturbance due to chemical change of the substance of the nerve fibre, that disturbance is probably only a minor part of the phenomenon. The electric flash passes at the rate of thousands of miles, the nerve current never faster than 200 feet, per second.1 This rate of transmission of a nerve impulse must how- ever be carefully distinguished from the time occupied by nerve centres in undergoing those changes which may or may not lead to consciousness or the perception of the sensation. Thus if it be arranged that a person shall make a signal as quickly as possible after seeing a flash of light, it is found that the time which elapses between the two events will be greater than would be required for the sensory impulse to pass to the sensory centre, and thence by efferent nerves to the muscles of the limb by which the movement is effected. There is time required for the supervention of the conscious state, and for the generation of the volition which leads to the movement. This interval has been called the psycho-physical time^ because we have here to do not merely with changes in nerve matter, but also with mental conditions and acts. The psycho-physical time varies considerably under different circumstances. Thus, for example, less time will be required if the observer has merely to make a prearranged signal that he has become conscious of some given sensory stimulus — the so-called perception time — than if he be asked to decide between two sensations, as of a low and high sound, 1 Recently it has been suggested that the nervous impulse is elec- trical, and that its velocity is slow compared with the velocity of elec- tricity, because great delay occurs at certain points along the fibre, known as the nodes of Ranvier. No positive proof has yet been offered of this somewhat fascinating theory. General Introduction or a bright or dull colour. This latter task requires nearly half a second of time. Even longer time is involved when the observer has to make a choice as to which of two stimuli he shall signal — somewhat more than half a second being usually required. On the other hand, it is possible that a stimulus to a sensory nerve may give rise to movement quite indepen- dently of consciousness and volition. In this case the sensory impulse affects certain nerve centres, either in the spinal cord or the base of the brain, which are able so to respond as to cause an efferent current to bring about some 6 c V FIG. i. — Various forms of cells, a, cylindrical or columnar ; <5, caudate or tailed ; c, fusiform or spindle-shaped ; d, ciliated, having fine filaments pro- jecting from their free surface ; e, stellate or branched. muscular action. In this case the time occupied in the nerve centre* is less than when volition is involved, but is, however, greater than would be required for the simple passage of the nerve current along a nerve. It amounts to about .05 of a second. Origin of Nervous System. — We have said that the nerves are largely composed of protoplasm. But this substance exists in all parts of the body, at least in early life. When we examine microscopically the tissues of the body during the earliest periods of its existence, we find that it is composed of minute vital elements to which the name of cells or corpuscles has been given. 8 Physiology of the Senses These cells are composed of protoplasm, and usually contain an exceedingly minute body, called the nucleus, whose composition is in certain respects different from that of protoplasm, and the cells may, moreover, be surrounded by a cell wall of less actively vital matter. At first the various cells of the body closely resemble one another, but as growth advances they become differentiated in form (Fig. i ) and structure in order to perform special functions, some cells going to build up the skin, some the muscles, some the nervous tissues and the like. In low forms of animal life, however, these cells are often not so highly differ- entiated as in man. Thus in the sea-anemone (Actinia), among the cells which go to form the outer covering or skin, we find certain cells from the free surface of which a hair -like filament projects, while from their attached border a number of processes pass inwards and join with like processes from other similar cells. These hair cells form rudi- mentary sense organs (Fig. 2). FIG. 2. — Neuro-epithelial cell from the upper nerve ring ,_, . . , of Carmina hastata. c, sense hair passing to the * U^^er, in the surface ; the two long thin processes join a ring of network formed nerve fibres containing ganglion cells. (Hertwig.) , , by the union of the processes just mentioned may be found cells which seem to have sunk inwards from the surface showing like processes, and regarded by Balfour J as an elementary sensory nervous apparatus. In general, it may be said that a study of the facts of development shows us that nerve cells appear at first upon the surface of the body, but that during the growth of the organism the cells become shut off from the surface; and in order to maintain their connection with the 1 F. M. Balfour, Comparative Embryology, vol. ii. p. 332. General Introduction 9 periphery, long processes called nerve fibres pass from the cells thus deeply embedded to the surface. Nerve cells may occur singly, or more commonly they are found gathered together in groups called ganglia, the individual cells being known as ganglionic nerve cells. These ganglionic cells are more or less closely connected with one another by means of nerve fibres, and thus community of action is established. In insects, for example, we find two rows of ganglia, the cells of which are united by nerve fibres both longitudinally and transversely. Sensory impressions pass by nerve fibres to these ganglia, and again, by other fibres passing out from these ganglia and ending in muscular tissue the move- ments of the body are regulated. In insects, too, it may be noted that the ganglia connected with organs of special sense, such as the eye or ear, are larger than the others. A further development of the nervous system arises through the fusion of ganglia with each other, so that the brain and spinal cord of vertebrate animals may be regarded as a vast number of ganglionic cells and nerve fibres bound into one consistent whole by a fine network of a connective tissue, and by an interlacing of nerve fibres. The nerve fibres connected with the brain and spinal cord may be divided, according to their function, into two sets — those which transmit sensory impressions inwards, the afferent nerves, and those which have to do with the regulation of such changes in the body as lead to motion or secretion, and known as efferent nerves. Thus the sensa- tion of pain, as, for example, toothache, originates from stimulation of a sensory or afferent nerve ; and the move- ments involved, say, in swallowing, from stimulation of efferent nerves passing outwards from the brain or cord. Structure of Nerves and Nerve Cells. — The progress of research tends to show that fibres of varying function io Physiology of the Senses always occupy a similar relative position in the central nervous system. As long ago as 1822, Majendie showed that the afferent or sensory fibres always pass into the spinal cord by what is known as the posterior root of a spinal nerve, while efferent or motor fibres emerge from its anterior aspect. See Fig. 3. But it has been found a matter of the greatest difficulty to determine accurately the course of fibres in the cord itself. When we look with the naked eye at a cross section of the spinal cord, we can see at a glance that it is made up apparently of two kinds of material, the outer part being whiter than the inner, which is l FIG. ^. — Portion of the spinal cord from the region of the neck, with roots of the nerves (slightly enlarged), i, i, The anterior median fissure ; 2, the posterior median fissure ; 3, the anterior lateral groove, from which the anterior roots of the nerves are seen emerging ; 4, posterior lateral groove where the pos- terior nerve roots enter the cord ; 5, anterior roots, to the right passing the ganglion ; 5', anterior root cut across ; 6, posterior root with ganglion at 6' ; 7, the nerve made up of anterior and posterior fibres ; 7', the first branches from the compound nerves. (Allen Thomson.) of a gray colour. This whiteness is due to the fact that the protoplasmic substance of the nerve fibre, the part which conveys, the nerve current, the so-called axis-cylinder of the nerve, is, in the greater part of its length, surrounded by a sheath of fatty material, known as the white substance of Schwann (Fig. 4), which in bulk gives a creamy white appearance to a group of nerve fibres. This, in turn, is enclosed by a thin transparent covering known as Schwann's sheath, or the primitive sheath. But in the central parts of the cord the white substance is to a large extent absent, and we here find among the fibres great numbers of ganglionic General Introduction ii nerve cells. These cells vary much in shape, but are mostly of the form called multipolar, on account of the large number of poles or nerve fibres which spring from them (Fig. 5), while others, and more especially the cells in the posterior part of the gray matter, are often spindle-shaped or jjyramidal (Fig. 6). These cells are in direct connection, for the most part, with efferent motor nerves ; and if they are destroyed by disease or otherwise, the nerve fibres with which they are connected quickly degenerate, and the parts supplied by them are paralysed. These are the cells which may be roused to action by the sensory nerves quite apart from any conscious sensation. If the foot of a person in profound sleep be lightly tickled, it will be drawn away without the sleeper being dis- turbed. If the middle or upper parts of the spinal cord be destroyed with- out injury to the lower part of the cord, while sensory impressions can pass to this lower part, and can set up changes in the nerve cells which lead to the movement of the lower FIG. 4. — Nerve fibres. B, part Of the body Or legs, these move- The axis - cylinder sur- ments are performed unconsciously, and therefore cannot be controlled or restrained by an act of will, since the impression is not transmitted to the brain. Man is only conscious when certain parts of his brain have been affected. Unless sensory impressions are transmitted to these parts, or unless these parts have been called into action by some variation in their chemical composition, there will be no consciousness. If these parts are ill- rounded by the white substance of Schwann, which is interrupted at A, a node of Ranvier, and contains a nucleus at C. The external line represents the primitive sheath or neurilemma. 12 Physiology of the Senses developed and ill -nourished, sensation will be feeble or perverted ; and if they are destroyed, the possibility of consciousness will be permanently lost. FIG. 5. — Multipolar nerve cells in the anterior part of the gray matter of the spinal cord, ar, anterior roots of emergent nerve fibres coming from the nerve cells, gc ', «/] nerve fibres cut across. PATHS OF NERVOUS IMPULSES i. The Spinal Cord. — When we seek the exact paths, how- FIG. 6. — Pyramidal nerve cells found principally in the brain. General Introduction 13 ever, along which sensory impulses pass up the cord to the brain, we are met by many difficulties. We can only infer that an animal feels some sensation ; we cannot enter into its consciousness of it. When the foot of an animal is pinched we believe that it feels pain because of some movement it makes, or some sound it utters, and because we know that a similar pinch to our own feet would cause a sensation of pain in us. But if, by careful and gradual operation, the greater part of the brain has been removed and the animal has survived, we find that the application of the stimulus may still educe movements or cries, while we cannot suppose the animal to be conscious of what it does. Another difficulty in the determination of the sensory path is that of isolating or destroying a certain part of the cord without injury to other parts, and without setting up irrita- tion or shock which may lead to erroneous inferences. It is impossible to reach the deeper parts of the cord without injuring the more superficial, and the individual fibres are so small that it is very much a matter of guess-work whether we have cut the parts we wish or not. We know that sensory fibres enter at the posterior part of the cord, that some of these fibres pass directly into the gray, some into the white, matter ; but hitherto it has not been possible to trace these fibres to any extent, on account of their bending away from the plane of section. It has been observed that at different stages of development certain strands of fibres are superposed, as it were, on others ; and by examining sections of cords of animals at different ages the connections of special tracts have been traced. Another method of study which has afforded valuable results is based upon the observation that when nerve fibres have been cut off from the nerve cells with which they are connected, the fibres quickly degenerate ; and thus it has been found possible to trace the line of de- 14 Physiology of the Senses generation for some distance. Similarly, in cases of loss of sensation in disease, it may be possible to discover, by post-mortem examination, the part which has suffered; but it will readily be seen that this, and the above- mentioned methods of research, can only afford rough and inaccurate results. One interesting fact we can con- clusively settle from cases of disease in the human being is, that different kinds of sensations travel by different paths in the cord. A lesion which may cut off the pos- sibility of feeling pain in a given part of the body, may leave it still susceptible to sensations of heat and cold ; or the sensation of touch may be present while the sensa- tion of pain cannot be aroused. From this we see that nerve impulses giving rise to sensations of touch, of pain, of temperature, of the muscular sense, must pass upwards to the sensorium by different paths, one of which may be cut off while the others remain. We may also learn from such cases that the sensory fibres, after passing up the cord, terminate in the opposite side of the brain from that in which we seem to have the sensation. Where the sensory fibres cross from one side to the other is not known. The experiments of the older physio- logists, and more especially those of the French observer, Brown -Sequard, seemed to show that the sensory fibres cross to the other side almost immediately after their entrance into the cord ; but later workers in this field of research maintain that the majority of the sensory fibres do not cross at once, but pass up almost to the base of the brain before they change sides. In some parts of the cord, however, the fibres do cross from the right to the left side, and vice versa, or decussate, as it is called ; so that sensory fibres from the right side of the body pass to the left side of the brain, and from the left side of the body to the right side of the brain. It is probable that they do not extend General Introduction 15 continuously, however, as single threads, from the peri- phery to the sensorium. We have seen that the stimulation of a sensory nerve, say in the right foot, may give rise to changes in the lower part of the cord, and hence to involuntary movements of which we are totally unconscious ; or it may cause a sensation by stimulation of the brain. Now we do not find nerve fibres branching except at their endings. Hence we are led to conjecture that the majority of the sensory fibres pass immediately into the gray matter of the cord and there become connected with nerve cells. From these some fibres may pass to the cells in the cord connected with efferent nerves, while other fibres pass upwards to the brain. To give a slightly more definite idea of the paths pur- sued by the different sensory fibres, we may refer to Fig. 7, in which we have a diagrammatic representation of a transverse section of the spinal cord divided into tracts or areas, which are to be understood as indicating bundles or columns of fibres running side by side and communicating freely with one another, but each containing, in the main, fibres of special origin and function. Thus, for example, the nerve fibres which convey painful impressions appar- ently pass into the gray matter of the cord, for if the gray matter be completely divided at any given level of the cord, there will no longer be a sensation of pain when the parts are injured which send nerve fibres to the cord below the level of section. From the gray matter fibres prob- ably pass outward and upward in the anterior root zone (ar, ar', Fig. 7). Suppose the gray matter were divided close above the region where sensory fibres from the legs pass into the cord. Then we might lacerate the foot, and though we might feel that it was being touched, we would have no sensation of pain from the operation. We distin- guish, therefore, between analgesia, or that condition in which painful sensations cannot be excited, and anaesthesia^ i6 Physiology of the Senses or the state in which we are insensitive to tactile sensa- tions. It will readily be understood that analgesia of any part of the body might lead to disastrous consequences. Thus among paralytics we find patients who feel no pain in, and are unable to move, the lower limbs. They will allow some part, such as the heel, to remain motionless on FIG. 7. — Transverse section of human spinal cord, ah, ah', anterior horns of gray matter ; ph, pti ', posterior horns of gray matter ; ar, ar1 ', anterior root zones ; pr,pr ', posterior root zones ; P, P', pyramidal fibres of lateral columns (mainly motor in function) ; T, columns of Tiirck (motor in function) ; G, columns of Goll ; dc, dd ' , direct cerebellar tract ; c, anterior commissure ; below c, central canal of cord lined with columnar epithelium. (Ross and Young.) a couch so long that the circulation of blood in it ceases, and its vitality may be seriously impaired. Similarly where the front of the eyeball has become insensitive to pain, the presence of small foreign bodies in the eye being no longer felt, such bodies accumulate in the eye, interfere with its well-being, and give rise to ulceration and de- struction of the ball. To the healthy body pain is nature's indicator of danger ; the burnt child dreads the fire. General Introduction 17 Tactile impressions in man pass upward, for the most part, in those columns of the cord which lie between the posterior roots of the spinal nerves. In this part, besides the paths for the stimuli which give rise to the sense of touch, we have probably also those which excite the sensa- tions of heat and cold, of pressure and resistance, and of tickling. That this is so is most distinctly shown by the study of changes in the cord during the progress of the disease known as locomotor ataxia — a disease, one promi- nent symptom of which is disorder of the power of walking. Patients subject to this disease usually suffer, in the earlier stages, from severe pains shooting apparently into the legs, and due to inflammatory changes in the posterior horns of the gray matter. Then the areas immediately adjoining these (pr, prj Fig. 7) become diseased, and the muscular sense is impaired, so that there is not the accustomed guide to the muscles as to the amount of force required for movement, and the patient tends to lift his feet too high and to set them down with a stamp. He is not able to judge accurately as to the weight of his limbs, nor of heavy masses attached to them. Then the delicacy of his sense of touch becoming impaired, he has the feeling, even when walking on rough ground, as if he were treading on velvet. No longer receiving the wonted guiding im- pressions from his feet, he must watch with his eyes his movements in walking, directing his steps by his sense of sight, and if he shuts his eyes he staggers and falls. His muscles act spasmodically, independently of each other, without due co-ordination. At first the motor power re- mains, but eventually it too may become involved, and the patient is paralysed for motion as well as sensation. In some animals, such as rabbits, it has been supposed that the tract for tactile sensations is in the lateral columns ; but all experiments on the sensory tracts are very apt to C 1 8 Physiology of the Senses be deceptive from the difficulty of interpreting the resulting phenomena. As the sensory tracts pass upward in the spinal cord they are somewhat modified in size and in relative position, owing to intercommunication and the entrance of fresh fibres, but on the whole the strands preserve the same general relationship. But just as the cord enters the cavity of the skull it enlarges, to form a portion about an inch and a quarter long, known as the bulb or medulla oblongata. Here the arrangement of the white and gray matter is much modified, and mixed with the fibres con- ducting nerve impulses to and from the brain we find several ganglionic centres which are of vital importance. Here, for example, we find centres which preside over the great functions of respiration and the circulation of the blood, besides such as regulate the acts of mastication and of swallowing, vocal utterance, the secretion of saliva and of sweat To these centres come efferent impulses from all parts of the body, impulses which may never indeed give rise to conscious sensation, but which, acting on the nerve centres of the medulla, so stimulate and affect them as to keep them constantly ready to respond to the needs of the organism. Under all the ordinary circum- stances of life, whether we be sleeping or waking, these centres pursue the even tenor of their way. Influenced by some great emotion, at some great crisis, when all the energy of our being is centred upon one thought or one swift effort, these centres may stand in abeyance for the moment ; nay, the pang may be so great that the vital chain is for ever broken, but as a rule we are unconscious even of the results of their activity. All the great vital functions go on unheeded, unless when some cause arises to interfere with their free and unimpeded action. But their influence over conscious life i§ none the less potent ; without their General Introduction 19 action the great receptive centres of the brain would be powerless. The freedom we have from the necessity of consciously watching over these things alone renders a higher life possible. 2. The Medulla. — The difficulties experienced in ascer- taining the paths of sensory influences in the cord are great, but they are vastly increased when we come to examine the medulla. We have, in fact, to depend mainly upon anatomical and pathological research for what little we know, and it is only possible to separate certain fibres which we can positively affirm to be associated with motor functions. The upward bound fibres passing through the medulla may either go to the ganglia at the base of the brain, to the cerebellum (Fig. 9, B), or to the cerebral hemispheres. A complete description of the structure and functions even of the parts of the brain devoted in the main to the sensory activities, is beyond the scope of the present work. We can only attempt to give a mere outline of the cerebral mechanism. 3. The Cerebellum. — The cerebellum, or little brain, is connected by strands of nerve fibres both with the cord and with the brain proper ; and though in all likelihood it acts as a co-ordinating or arranging centre for the nerve currents that induce complicated movements, we have no evidence that it contains any sensory centres. No pain is felt when its sub- stance is injured, nor can we detect any alteration in general or special sensitivity. Some physiologists have advanced the view that it may be connected with the muscular sense. The staggering gait and irregular movements characteristic of an animal whose cerebellum has been destroyed, indicate a loss of a regulating centre which normally is at work. We may understand this if we reflect for a moment upon the complicated nature of the movements we habitually 20 Physiology of the Senses perform. Walking, for example, involves the co-ordinated action of many groups of muscles, each of which must act exactly at the proper time and with most delicately adjusted force. The acquirement of the power is only gained after many attempts, and the mere preservation of the upright attitude of the body is only possible when the sensory impressions from the feet and limbs are duly transmitted and take their place in the complex sum of afferent impulses. Of the means or methods by which the multifarious peri- pheral impressions are correlated, and after the nerve centres are excited, the adjustment is carried out and the different muscles set in regulated motion, we know nothing. We do not even think how a movement is to be made. We simply will something to be done, and it is done ; but of the intervening causal chain we are quite unconscious. We think of the end and not of the means. In that sense our movements are automatic ; and it is interesting to note that the more any given movements are practised, the more auto- matic they become ; and the more purely automatic they are, the more accurately are they adapted to their aim. Illustra- tions of this are afforded us in all employments where a certain small piece of work is done to the exclusion of all else. The hands will work busily while the thoughts are far away. In such a case we have the same sensory im- pression travelling to the same centre, giving rise to the same outflow of energy, and along the same efferent channels, and an unconscious memory of what has been required in the past enables us to determine without effort the neces- sities of the present. But vary the surroundings a little, and new conscious efforts must again be made, and the work requires longer time and conscious effort and attention. It is possible that the necessary fusion of impressions takes place in the ganglia at the base of the brain, and messages to the cerebellum act through its cells and fibres as through General Introduction 21 distributing centres to the muscles ; but of this we cannot at present speak with certainty. 4. The Pons. — The medulla, as we have seen, is con- FIG. 8. — Base of the brain, i, i, The longitudinal fissure dividing the hemi- spheres ; 2, 2', 2", the anterior lobe of the brain ; 3, fissure of Sylvius ; 4, 4', 4", the middle lobe of the brain ; 5, 5', posterior lobe ; 6, bulb or medulla oblon- gata ; 7, 8, 9, 10, the inferior surface of the cerebellum. The figures I to IX indicate cerebral nerves : thus I is the olfactory bulb removed on the right side ; II is the optic nerve with decussation ; V, the sensory nerve of the face and part of the scalp; VII, the auditory nerve; VIII, the glosso- pharyngeal with sensory fibres from mouth and throat ; III is on a crus cerebri ; VI and VII are placed on the Pons Varolii ; X, the first nerve emergent on the neck. nected with the cerebellum ; the rest of the fibres passing upwards from it enter a structure known as the pons Varolii j 22 Physiology of the Senses or bridge of Varolius (Fig. 8, VI, VII ; Fig. 9, C), so called because numerous fibres pass through it from one side of the cerebellum to the other, and these form a transverse prominence like a bridge across the main course of the nerve fibres which pass up and down. In the pons, as in the medulla, we find many nerve centres mixed with the fibres. Here, for example, among others are situated the centres of origin of the great nerve — the fifth cranial (Fig. 8, V), or main path for general sensory impressions from the face and scalp, of the auditory nerve (Fig. 8, VII) coming from the ear, and of the nerves which control the movements of the muscles of the face. Fibres carrying painful, thermal, and tactile impressions probably pass up through the centre of the pons, where also some of them decussate. The motor fibres are mainly in front of, and the nerve centres behind, these thermal and tactile paths. 5. The Cerebrum. — Fibres from the pons and cerebellum pass to the cerebrum, or brain proper, by the connecting strands known as the cerebral peduncles. These slope upwards and forwards, and the anterior and lower fibres branching outward as they enter each side of the brain are known as the legs of the brain, or crura cerebri. The upper and back part of the peduncles is composed mainly of gray matter, and when seen from above shows four slight elevations known as the corpora quadrige- mina. It is of interest to note that the corpora quadrigemina receive nerve fibres from the eyes through the optic tracts, and are concerned in the mechanism of vision. Destruction of one side causes blindness in the eye of the opposite side, with loss of power of accommoda- tion of the pupil of the eye. Whether they are the seat of conscious sensation is, however, very dubious. They are small, and hidden away under the superposed cerebral mass in man, but the corresponding structures General Introduction 23 known as the optic lobes in birds, reptiles, and fishes are large and important relatively to the rest of the brain. The most attractive hypothesis is that they act in man as centres for the fusion of impressions coming from the eyes by the separate nerve fibres, and for the regulation of bodily or ocular movements dependent upon visual impressions, but FIG. 9. — Plan In outline of the encephalon, or central nerve system within the skull, as seen from the right side. A, Cerebrum ; B, Cerebellum ; C, Pons Varolii ; D, Medulla oblongata ; «, crus cerebri or cerebral peduncle ; b superior, ~c middle, d inferior cerebellar peduncles ; b is placed just in front of the corpora quadrigemina ; e, fissure of Sylvius ; f anterior, g middle, k posterior lobes of cerebrum. that for conscious vision the gray matter of the cerebrum must be likewise affected. In front of the corpora quadrigemina^ and lying at the base of the brain, lie two large ganglionic masses on each side of the middle line — the thalami optici and the corpora striata — between which passes an important set of fibres from the crura, known as the internal capsule. Many 24 Physiology of the Senses sensory fibres are believed to enter the optic thalami, coming either by way of the corpora quadrigemina, the crura, or the internal capsule, while other fibres join the thalami with the cerebral hemispheres. From their con- nection with the corpora quadrigemina we find, as might have been expected, that injury to the optic thalami, more especially in their hinder parts, causes visual disturbance, but the thalami are probably connected with many other sensory fibres besides those of vision. The human brain, when stripped of its investing mem- branes and viewed from above, is seen to consist of two masses or hemispheres of a grayish colour externally, a deep furrow running between the hemispheres from before backward, at the bottom of which is a broad band of white nerve fibres, the corpus callosum, joining the two masses. The surface is not smooth, but thrown into numerous folds, convolutions, or gyri^ between which lie depressions of vary- ing depth called sulci, or fissures. Such convolutions are absent from the brains of many of the lower forms of animals, and even in man, in the earliest periods of life, and they are present in the adult brain in order to allow for increased area of the cerebral surface or cortex. At a first glance these ridges and furrows seem to be quite irregular and devoid of arrangement, but a study of the comparative appearances of many human brains leads us to see that though there may be slight divergencies in the number, depth, and regularity of the convolutions, these are largely formed on the same plan. We see that the brain may be regarded as made up of several lobes (Fig. 10), which are named according to the part of the cranium in which they lie, and that each lobe has a definite number of ridges and furrows, the names of which are given in the explana- tion of Fig. 10. So long as it was supposed that the brain acted as a whole, and that no special functicn was associated General Introdttction 25 with any particular area, the relationship of the convolutions was deemed of comparatively little importance. Now, F P FIG. 10. — Semi-diagrammatic view of the left side of the brain. F, Frontal lobe ; P, Parietal lobe ; O, Occipital lobe ; T, Temporo-sphenoidal lobe ; S, fissure of Sylvius ; S' horizontal, S" ascending branch of the same ; c, central sulcus or fissure of Rolando ; A, ascending frontal ; B, ascending parietal convolu- tion ; FI, FZ, FS, superior, middle, and inferior frontal convolutions;^,^, superior and inferior frontal sulcus ; f%, prsecentral sulcus ; Pj, superior parietal lobule ; P% supra-marginal gyrus, and Pg' angular gyrus, parts of inferior parietal lobule ; ifi, intra-parietal sulcus ; cm, end of calloso-marginal fissure (see Fig. n); Oj, Os, OB, first, second, and third occipital convolu- tions ; po, parieto-occipital fissure ; o, transverse occipital fissure ; 0%, inferior occipital fissure ; TI, T%, T$, first, second, and third occipital convolutions ; tl, t2, first and second temporo-sphenoidal fissures. (Ecker.) however, it is well to know the names and positions of the various lobes, convolutions, and furrows, so as to be able to 26 Physiology of the Senses understand descriptions of special areas of the surface. The lobes are named from the special bones of the skull with which they come into contact, and are known respec- tively as the Frontal, F, the Parietal, P, the Occipital, O, and the Temporo-sphenoidal, T, lobes. It will be seen by reference to Fig. 10 that there are two specially deep and well-marked fissures, those of Rolando (c, Fig. 10) and of Sylvius (S, S', Fig. 10), the latter of which is branched, S". To the front are three well-marked and constant ridges, the frontal gyri (F1? F2, F3), separated by two furrows, fv fy In front of the fissure of Rolando we have the ascending frontal convolution, and behind it the ascending parietal, behind which again, and separated by the intra-parietal furrow, lie two other parietal convolutions, Pj and P2. The second parietal convolution becomes continuous with the superior of three temporal convolutions, Tp T2, and T3, by a bend round the end of the Sylvian fissure immediately below P2, known as the supra-marginal convolution, and the superior and middle temporal convolutions are connected posteriorly by a small angular convolution at P2 , commonly known as the angular gyrus. Parts of three occipital con- volutions, O15 O2, O3, are seen. Of the various fissures that of Sylvius is much the most marked, the others being merely furrows. The Sylvian fissure really indicates that the posterior part of the hemi- sphere has in the process of development been bent round and packed away under the frontal and parietal regions. When the Sylvian fissure is opened up there is seen a small pyramidal mass of gray matter — the island of Reil — the convolutions of whose surface, being hidden when the brain is in its natural state, are known as the gyri operti. The letter S lies external to the spot in which these convolu- tions are to be found. When the two hemispheres are separated by an antero- General Introduction 27 posterior section in the median plane of the body, each internal surface is seen to present certain fissures and con- volutions, the principal of which are — (i) the marginal gyrus F.p which is really the internal aspect of the superior and ascending frontal convolutions and ends posteriorly at the fissure of Rolando ; (2) the gyrus fornicatus^ Gf, 1892, p. 73. The Sense of Smell 95 respiratory movements.. Thus the smell of winter green notably increased the respiratory work ; next came ylang- ylang ; and last rosemary. The breathing of a fine odour is therefore not only a pleasure, but it increases the amplitude of the respiratory movements. Just as taste and flavour influence nutrition by affecting the digestive process, and as the sight of agreeable or beautiful objects, and the hearing of melodious and harmonious sounds, react on the body and help physiological well-being, so the odours of the country, or even those of the perfumer, play a beneficent role in the economy of life. THE SENSE OF SIGHT ™ Eh FIG. 29. — Antero-posterior section through upper eyelid, X 7 d. i, Outer skin — E, epidermis ; C, corium; Sc, subcutaneous tissue; H<5, fine hairs; K, M, sweat glands ;,W, eyelash ; W',W", roots of eyelashes ; E^, reserve hair ; 2, muscles for closing eye — O, muscular bundles cut trans- versely ; McR, ciliary muscle of Riolanus ; 3, tendon of muscle elevating the eyelid, mps ; 4, conjunctival region ; tp, tunica propria ; e, con- junctival epithelium ; at, gland ; t, tarsus ; m, Meibomian gland ; a, a', arteries ; 5, corner of eyelid. (Stohr.) THE sense of sight differs from the senses of taste and smell in this im- portant particular, that through it we seem to be- come aware of the exist- ence of things which are entirely apart from us, and have no direct or material link connecting them with our bodies. Yet physi- cists tell us that in vision the eye must be affected by a something which is as certainly material as a sapid or an odorous sub- stance, and which, per- meating the universe, transmits by its vibrations movements that affect the eye, and give rise to the sensation of light, or to the perception of even the most distant objects. This medium for the transmission of light is The Sense of Sight 97 called the luminiferous ether, and our eyes are so constituted as to respond to its vibrations ; changes are set up in the optic nerve and in the brain, and we see. That the eye may be sufficiently sensitive to the ray of light, its sensory surface must be carefully protected from all hurtful influences. Accordingly, we find that the eyeball, embedded in soft fat, is placed in a socket whose margins are formed of strong bone which can withstand heavy blows ; it is also protected from drying by the action of the lachrymal gland which secretes a watery fluid, and from dust and foreign bodies by the lids with their long eyelashes. The watery fluid which bathes the eyes passes away by two fine pores at the inner angles of the eyelids into a passage to the nose, and is prevented from overflow- ing and running down the cheeks by an oily secretion coming from glands in the upper eyelid (Fig. 29, m) which anoints the edges of the eyelids (Fig. 29). Furthermore, the eyebrows protect the eyes from perspiration trickling from the forehead. The eye may be moved in various directions by muscles which will be described later. I— STRUCTURE OF THE EYE Coats of the Eyeball. — The eyeball is nearly spherical in shape, but is slightly elongated from before backwards, for the front part, which is clear and transparent, to allow the entrance of the rays of light, bulges forward somewhat prominently. The ball is elastic but firm, and is enclosed by a covering which may be divided into three layers, each of which has important functions to discharge. (For the relative position of the various parts of the eyeball see Fig. 30.) I. The outermost coating is composed of a laye of firmly H 98 Physiology of the Senses felted fibrous tissue, which, being very tough, preserves the form, and prevents rupture of the eyeball. To it the muscles that move the eyeball are attached. It is called the sclerotic (Greek, scleros^ hard), and the part of it seen when the eye is open is known as the "white of the --17 FIG. 30. — Diagrammatic section of the eyeball, i, Sclerotic ; 2, junction of sclerotic and cornea ; 3, cornea ; 4, 5, conjunctiva ; 6, posterior elastic lamina ; 7, junction of iris with choroid ; 8, canal of Schlemm, a lymph space ; 9, pigmented tissue uniting sclerotic to choroid; 10, choroid; n, 12, 13, ciliary processes ; 14, iris touching, but not connected with lens posteriorly ; 15, retina lined by hyaloid membrane ; 16, optic nerve ; 17, central artery of the retina ; 18, yellow spot with central groove ; 19, 20, anterior portion of retina ; 21, junction of choroid and ciliary processes ; 23, free border of ciliary process resting on anterior suspensory ligament of lens ; 22, canal of Petit ; 24, hyaloid membrane ; 25, fibres to posterior surface of lens ; 26, 27, 28, lens ; 29, vitreous humour ; 30, anterior chamber containing aqueous humour ; 31, posterior chamber communicating with 30. eye." In early childhood the white of the eye, being thin, appears bluish in tint from the pigment seen through it, while in old age it becomes yellowish by a deposit of fat. The clear transparent circular disc in the front of the eye, the cornea, is a modification of this external coat. The The Sense of Sight 99 jy~ n fibres of the cornea are united by a cement substance into transparent sheets or membranes, which lie parallel to one another like the coats of an onion, but connected together by many intercommunicating fibres (Fig. 31). In the flat spaces between the fibrous sheets lie numerous corpuscles, flattened, transparent, and branching so as to join with one another. The fibrous sub- stance of the cornea is lined in front and behind by a homogeneous elastic layer, that at the back of the cornea being the thicker and called the posterior elastic lamina of Bowman, or the membrane of Descemet. This lamina is itself covered on its posterior aspect by a layer of flattened cells lying side by side as in a tesselated pavement. There are no blood-vessels in the cornea, nutrition being effected through the branching cells. The whole of the exposed part ryqh3W*i nerve sending branches to Conjunctiva, which is COntinUOUS all cornea and conjunctiva ; f, . , , ... , .., fibres of cornea between which round with that lining the eyelids, are flattened spaces containing and which, closely adherent to the cornea, and more loosely joined to the sclerotic, forms a sensitive pro- tective covering for the open eye. 2. The middle coat, the choroid, is largely composed of blood-vessels which branch frequently in its outer part, corpuscles ; d, layer of cells covering posterior surface of cornea, and separated from the fibrous part by the posterior elastic membrane. (Schofield.) ioo Physiology of the Senses and form a very fine network of capillaries to the inside. The blood-vessels of the choroid coat are known as the ciliary arteries and veins. The veins as they emerge join together in a stellate fashion, forming groups, the vena vorticosce, from the union of which single veins pass out- wards through the sclerotic. The spaces between the vessels are occupied by elastic fibrous tissue, and by cells loaded with granules of very dark brown pigment, the whole being bound together by cement substance. The colouring matter renders the choroid. opaque, and absorbs" the rays of light pass- -a ing into the eye, thus preventing their reflec- tion to and fro in the interior of the eyeball, and the confused vision that would ensue there- from. FIG. 32. — Antero - posterior section through _,, , . , . conjunctiva and fore part of human cornea, rhe choroid IS closely X24od. i, Conjunctiva ; a, nerve fibres in united tO the Sclerotic conjunctiva ; s, network of nerve fibres be- , - tween conjunctiva and cornea; 2, anterior bY meanS °f Connective. elastic membrane ; 3, substance of cornea tissue, but just where with n, a nerve passing through it. (Stohr.) the sclerotic merges into the cornea an interesting and important alteration occurs. Were the choroid to line the cornea as it does the sclerotic, light could not enter the eye. Accordingly this coloured layer hangs separate from the cornea as a curtain or ring of variable size called the iris (iris, a rainbow), and is pierced by an aperture known as the pupil, through which light may enter. The space between the iris and the cornea, the anterior chamber, is filled with a watery fluid, the aqueous humour. The back of the iris is lined with dark pigment, and according as the substance of the iris con- tains less or more pigment, the eye has a blue, gray, or The Sense of Sight 101 brown colour. The central aperture is usually black, from the pigment absorbing most of the light that enters the eye, so that almost none is reflected out again ; but sometimes, as in albinos, the pigment is awanting, and then the pupil is pink, as may be seen in white rabbits. In many of the lower animals the pupil is often seen of a greenish lustre owing to partial reflection of light from the back of the eye, In herbivora this iridescent gleam is due to the arrange- ment of the fibres to the outside of the capillary layer in a 10 FIG. 33. — Meridional section through ciliary region of human eye, X 20 d. i, 2, Epithelium and loose connective tissue of conjunctiva ; 3, sclerotic ; 4 meri- dional, 5 radiating, and 6 circular fibres of ciliary muscle ; 7, ciliary process ; 8, ciliary part of retina ; 9, pigmentary layer on the posterior surface of the iris; 10, the iris; u, the posterior elastic lamina; 12, the cornea; 13, con- junctiva ; 14, canal of Schlemm ; 15, in the anterior chamber points to junc- tion of iris with sclerotic. (Stohr.) structure called the tapetum, while in carnivora and birds of prey it is brought about by reflection from cells which contain minute crystals and act like prisms. The amount of light, moreover, which enters the eye is regulated by variation in the size of the pupil. There are con- tractile fibres radiating in the iris like the spokes of a wheel, and when these contract the pupil dilates. On the ether hand, if too much light is entering the eye, a circular band of muscle fibre in the iris, near the margin of the pupil, 102 Physiology of the Senses contracts, and the pupil is lessened in size. The iris if joined to the sclerotic by muscular as well as by connective tissue. The muscular fibres are disposed, partly so as to radiate from the junction of the cornea and sclerotic to that of the iris and choroid, and partly to form a ring round the outer border of the iris, as seen in Fig. 33. Together they form what is called the ciliary muscle, and this assists largely in accommodating' the eye for the perception of objects at different distances. Just behind the ciliary muscle lies a curious modifica- tion of the choroid, consisting of a ring of tooth-like tufts, of capillary blood- vessels, bound together by connective c tissue, and pointing towards the pupil. These are the ciliary processes. The , choroid and ciliary processes are lined internally by a thin transparent mem- a brane, known as the membrane of Bruch. 3. The innermost coat, the retina^ FIG. 34. — Blood-vessels of the choroid and iris of the human eye seen from is the terminal organ of vision, and within, a, Capillary ves- sels of the choroid : b. is almost transparent, with a pinkish serrated line of union of tinge, except at a point in the visual choroid with ciliary pro- • IIJ-L /r r i • cesses;,, veins of dlLy EX1S Called thc.Jjrffow SJ0t, of which ring; d, capillaries of more anon. The retina contains the iT^rn^'pt ^rminal branches of the optic nerve, of iris ;f, vessels of pupil- which, piercing the sclerotic and laryzoneof iris. (Arnold.) choroid jn the human £ye ^ & ^ about y1^ of an inch nearer the nose than the antero-posterior axis of the eye, and forming an oval area known as the optic pore, spreads out in nerve fibres ramifying over all the The Sense of Sight interior of the eye as far forward as the ciliary processes. These nerve fibres are the more transparent as theyare simply axis cylinders, devoid in the retina of the white substance of Schwann. They are supported by connective tissue which is found in most parts of the retina as fibres passing radi- ally, fa& fibres of Milller. The connective tissue also forms external and internal limiting membranes and a fine net- work through the substance of the retina, keeping the various elements in their proper places. Small blood- vessels are also found in the inner layers of the retina. J. Pigmentary layer not seen. Layer of rods and cones. • External limiting membrane. • Outer nuclear layer. / ^~ 5. Outer reticular layer. " ,1 6. Inner nuclear layer. i "7- Inner reticular layer. Ganglion cell layer. , Nerve fibre layer. FIG. 35. — Vertical section of human retina, X 240 d. £, Blood-vessel ; k, conical base of radiating sustentacular fibre of Miiller. The base of several fibres uniting gives rise to the appearance of an internal limiting membrane. (Stohr.) After spreading over the fundus or concavity of the retina, the nerve fibrils turn outwards and become con- nected with a set of ganglionic cells (see Fig. 36), from which, again, fibres may be traced outwards for a certain distance. These fibres are believed to become connected with nuclei, which are found in two layers to the outside of the ganglionic cells, and from the outer layer of nuclei fibres pass to the true terminal sensory organ, the so-called Jacobs membrane or layer of rods and cones. This layer lies outside of and upon the external limiting membrane. Physiology of the Senses The rods and cones consist alike of an inner and an outer part. In the cones, the inner part is thick and conical, and exhibits a longitudinal striation (Fig. 37) ; in the rods it is thinner : both are connected with nucleated fibres, internal to the outer limiting membrane. The outer part of the rods FIG. 36. — Diagram showing retinal elements. Two fibres of Miiller with expanded bases at a, pass outwards as fine cylindrical processes, giving off slender lateral twigs (not shown in' diagram) in the reticular layers d and./J and forming meshworks in the layers e and g. The spaces of the mesh work are occupied by nuclei. The fibres terminate in the external limiting membrane h. Opposite c two ganglionic cells are seen, their inner processes continuous with optic nerve fibres in b, their outer processes breaking up into numerous twigs in d. The nuclei of the layer e belong partly to the fibres of Miiller, partly to cells which send many branching processes to the outer and inner reti- cular layers, and probably establish functional continuity between the ganglion cells and the rods and cones. The nuclei of g are surrounded by a thin layer of protoplasm, and are connected externally with the rods and cones by processes perforating the external limiting membrane, and internally by fine fibres known respectively as rod and cone fibres, with the network of the cuter reticular layer. The nuclei connected with the rods show one or two transverse dark bands. The rods and cones of the layer z'show the differentiation into an outer and inner limb. The outer limb of the cone is shorter than that of the rods. (Zehender.) is of a pink colour, and considerably longer than that of the cones, but both exhibit a transverse striation, and, under the influence of macerating reagents, tend to break up into highly refractile discs. The rods are much more numerous than the cones, but the fore part of the retina has cones The Sense of Sight only, while the part of the retina lining the iris has neither rods nor cones. On the other hand, in the yellow spot above mentioned we find cones but no rods. Here; too, we find the layer of ganglion cells at first thickened, but soon thinning, and there is formed in the centre of the yellow spot a short groove or depression, the fovea centra/is, where the various layers of the retina above described disappear, and we find only a layer of cones with the fine terminations of the nerves. This spot is the seat of most distinct vision. Outside of, and in apposition with, Jacob's membrane lies a layer of hexagonal cells, containing, more espe- cially on their inner side, a vast number of pigment granules of a brown colouring matter called fuscin or melanin. Under the action of light, the cells send pro- cesses carrying the pigment inwards between the outer segments of the rods and cones, and thus absorb the rays of light after they have passed through the retina. If the eye is kept in darkness for some time, these processes are with- drawn into the main bodies of the cells, and the layer of pigmented epithelium may then be easily detached from the adjoining layer of the retina (Fig. 39). Contents of the Eyeball. — Inside of, and closely adherent to, the retina we find a perfectly transparent, highly elastic bag called the hyaloid membrane (hyalosy glass), which might be compared to the membrane lining the shell of an egg. This bag is filled with a transparent FIG. 37. — Diagram of rods and cones, show- ing faint longitudinal striation of inner limbs of rods and cones, and varicosities of the rod- fibres. (Max Schultze.) io6 Physiology of the Senses glassy-like jelly, like white of egg, called the vitreous humour (Fig. 30, p. 98), and composed of fluid, penetrated in all direc- tions by fine fibres and a few connect- ive tissue cells. In front, the hyaloid membrane closely adheres to the circle of ciliary processes but not to the iris, and it splits into two layers or suspen- sory ligaments, which are attached to a capsule in which lies the crystalline FIG. 38.— Rods and cones lens- The suspensory ligament forms seen from without on a rjng called the zonule of Zinn> and removal of pigmentary layer. The larger circles bounded by the two layers and the lens represent the inner limb is a triangular space containing fluid, of the cones ; the smaller , , , , , central circles, the outer and called the canal °f P'M- The limb of the cones, in 2 ligament, it may be noted, is much and 3, the cones are sur- r.jurii • T- » • rounded by rods. i.From PllCated bY following the Convolutions the yellow spot ; 2, from of the ciliary processes, and the pos- 3' terior layer is perforated with numerous apertures (Fig. 30). The lens is composed of fine flattened fibres hexagonal in cross section, and with serrated edges which fit exactly into one another, and are bound together by a kind of cement substance. The fibres run in an obliquely meridional direction (see Fig. 41, C), not forming a Complete Semicircles from FlG' 39- - Hexagonal pigmented cells pole to pole, but fixed at their ends to a tri-radiate mass of cement substance, whose rays form angles of 120° with one another, and, as they pass through the sub- stance of the lens, are rotated like a wheel in motion covering J acob s membrane, a, Surface- view ; b, cells seen from the side, sending ^ Processes between rods and cones. 1 he lighter portion in the centre of the cells in a, indicates the non-pigmented nudeus> (Max Schullze-) The Sense of Sight 107 through an angle of 60°. The lens, like the capsule which holds it, is perfectly clear and transparent. Should it become opaque, we have the disorder known as cataract. It has a bi- convex form, its front sur- face being somewhat more flattened than that behind, bu't it is highly elastic, and the curves are constantly changing as the eye is accommodated for near and distant objects. The capsule surrounding the lens is very thin and elastic, and, by the tension of the anterior suspensory liga- ment, the surface of the lens is kept slightly flat- tened. In its earliest stages of development, the lens is formed by an invagina- tion or growth inwards of a process of the deepest layer of the epidermis, which is cut off as a closed sac. The central cavity is obliterated by the elonga- tion of the cells at the back of the sac, the cells in front remaining small and cubical, and forming the FIG. 40. — Lens fibres. A, From eye of ox showing serrated edges ; B, cross section of lens fibres from human eye ; C, fibres from the equatorial region of the human eye. The fibres are seen edgewise except in A and at C, 2. Near i, nuclei of lens fibres. (Schwalbe, after Kolliker and Henle.) io8 Physiology of the Senses anterior epithelium of the lens. The lens may be artificially broken up into a set of concentric layers (Fig. 42), in which the fibres run in a meridional direction, and the outer layers are softer and more gelatinous than those towards the centre. The lens from the eye of a lightly boiled fish affords con- venient material for the study of the structure of the lens. It appears as an opaque white ball, but when the outer part is detached with a knife an inner translucent core is found, from which thin transparent sheets may be readily peeled FIG. 41. — Diagram of arrangement of lens fibres. A Posterior, B anterior, and C lateral view, c, in each figure, indicates the centre of the tri-radiate cement substance. The numbers i to 6 indicate the same six lens fibres, the course they take being seen by comparison of the figures. (Allen Thomson.) off and broken up into fibres. The iris, to have perfect mobility, hangs free, not only of the cornea in front, but also of the lens and its suspensory ligament behind, except in its central part round the pupil, where it rests lightly on the lens. The space behind the iris and in front of the lens and suspensory ligament is called the posterior chamber. This is filled with fluid, which is similar to, and in com- munication with, the aqueous humour in the anterior chamber. We thus see that the contents of the eyeball are all transparent, and light traversing the eye must pass first The Sense Q/ Sight 109 i, The denser central part ; 2, 2, 2, concentric outer layers. (Arnold.) through the conjunctiva and cornea in front, then through the aqueous humour, thereafter through the lens with its capsule, and finally through the vitreous humour and the hyaloid mem- brane. The Optic Nerve. — The nerve fibres converge from all parts of the retina to the optic pore, and there passing through , . , FIG. 42. — Laminated structure of a membrane in which are many the crystalline lens. Thelamin* fine Openings for their passage, are split up after hardening in alcohol. the lamina cnbrosa, they are grouped together into a bundle forming the optic nerve. The \jptic nerve from each eye passes bac!:v.T.rds, and entering the hollow of the cranium by a passage at the back of the orbit, joins with its fellow in a union called the optic commtssitre. At the commis- sure some of the fibres pass directly upwards into the brain, but in the human eye the most of the fibres from the inner or FIG. 43. — Course of nerve fibres in posterior part of nasal half of each retina, i, Optic pore ; 2, yellow spot (macula lutea) ; rgfjna deCUSSate 3, fibres to yellow spot. (Schwalbe.) or in other words cross over, and pass backwards to the half of the brain no Physiology of the Senses opposite to the eye from which they have come, while fibres from the outer or temporal (next the temples) side of each retina pass back to the brain on the same side as the eye from which they have sprung. Hence it will be seen that almost all the fibres affected by rays of light which come from objects on the left side of the body (a, Fig. 44) will transmit impressions to the right side of the brain, while luminous impressions from the right side of the eyes will be transmitted to the left half of the brain. The bundles of nerve fibres continued behind the optic commissure are known as the optic tracts, and they pass to certain ganglia at the base of the brain, from which again fibres pass to the occipital or posterior part of the cerebral hemispheres, the stimulation of which gives rise 7,>>~r7fl to a sensation of light. But the eye is in connec- tion with other nerve fibres •/•// \v\^\ besides those of the optic nerve. FIG. 44.-Diagrammatic 'represents We a11 kn°W h°W Sensitive tion of decussation of fibres of the the eye is to touch, and how opticnerves' acutely painful is any lesion of the eyeball. Impulses giving rise to tactile or painful sensations are sent to the brain through the medium of branches of a nerve known as the ophthalmic division of the fifth cranial, or great sensory, nerve of the head, from which there also pass to the iris several branches known as the long ciliary nerves, to whose function reference will shortly be made. Again, the eye, as a whole, and certain parts within the eye, can be moved under the influence of muscular contrac- tion, and to effect these movements we have the oculo-motor or third cranial nerve, and the fourth and sixth cranial nerves, The fibres of the third cranial which supply the The Sense of Sight in sphincter of the iris pass through a ganglion known as the ciliary ganglion^ where they meet with fibres from the sympathetic system, and a branch from the ophthalmic nerve. From the ganglion a large number of twigs, the short ciliary nerves, pass to the back of the eyeball, where, having pierced the sclerotic coat, they run forward between the sclerotic and choroid coats to the ciliary muscle, the iris, and the cornea. Stimuli pass by the short ciliary nerves, as a result of which the pupil may vary in diameter, or the eye be accommodated for the perception of objects at vary- ing distances. Movements of the Pupil. — Various influences may cause change in the size of the pupil. The brighter the light entering the eye, the nearer the object we look at, or the more we converge the two eyes, the more the pupil contracts. In certain stages of poisoning by opium, tobacco, alcohol, chloroform, and physostigmin, in sleep, or in unconscious states as during an epileptic fit, the pupil may be contracted to a mere pin-hole aperture. Dilation of the pupil occurs when the light is dim, when the eye is looking at distant objects, when respira- tion is obstructed, or the body strongly stimulated ; under the effect of certain drugs, such as belladonna, or its active principle atropin, by Indian hemp or hyoscyamin ; in the later stages of poisoning by alcohol, chloroform, and other substances ; and under the influence of mental emotions, such as fear. This change in size of the pupil is an involuntary move- ment, and goes on without consciousness upon our part, unless we are directly observing it in a mirror. It is of the nature of a reflex act. The usual exciting cause of the movement is a variation in the amount of light entering the eye, and a consequent variation of the amount of stimulus to the optic nerve. If the optic nerve is cut, or if the 1 1 2 Physiology of the Senses centre to which it passes in the brain is destroyed, the pupil no longer contracts when light falls on the retina, although the oculo-motor or short ciliary nerves may still be directly stimulated by electricity or mechanical irritation, so as to cause contraction. Moreover, the third nerve con- tains at least two sets of fibres, stimulation of one of which causes contraction of the pupil, of the other, movements of accommodation, and, as might be expected, these fibres originate in different centres in the brain. These centres are situated close to each other in the basal ganglia, and on a lower level than the cortical centres involved in conscious vision. The pupil is caused to dilate by stimulation of the sym- pathetic nerve which, coming from a ganglionic centre situated in the neck, and having entered the cranial cavity, becomes apposed to the ophthalmic nerve, and is given off to the eye from its nasal branch as the long ciliary nerves. There has been much discussion as to its mode of action, but apparently it supplies the dilating muscular fibres of the iris. The oculo-motor to the sphincter of the iris, and sympathetic to the dilating fibres of the iris, would thus seem to act as antagonists to each other. Moreover, they seem to keep up a constant balancing tonic action, because if one is injured the other immediately shows its power. For instance, if the sympathetic fibres be cut, the pupil will at once contract, and vice versd. But this is merely a particular instance of the general law which regulates the condition of the muscles of the body, so long as their nerve supply is normal and in healthy action. Another point of interest in regard to the human eye is that a strong stimulus to one eye will cause contraction of both pupils. This is probably due to the incomplete decussation of the optic nerves, the fibres from one eye passing, as we have seen, to centres on both sides of the brain j for in animals that The Sense of Sight 113 have a complete decussation, and want the power of binocular vision, this phenomenon is absent. We should note in passing that the foregoing explanation of the mechanism of contraction and dilation of the pupil has been called in question by some physiologists. They deny that the so-called dilator of the iris consists of true muscular tissue at all, and maintain that the sphincter action of contraction is the only really muscular act. Dilation is attributed to elastic recoil, the sphincter being held to be inhibited or thrown out of action by stimulation of the sympathetic. When the pupil contracts, the elastic radiat- ing fibres are stretched ; when the muscle ceases to act, elasticity comes into play, and the pupil dilates. Recent observations seem to show that changes in the calibre of the blood-vessels of the iris, brought about by nervous action, are not the cause of variations in the diameter of the pupil. The iris of birds contains specially developed striated mus- cular fibres, and a more careful examination of such eyes may yet throw light upon this problem. Drugs may act either directly upon the muscles of the iris, or indirectly through the nerve centres. Thus, even in an eye removed from the body, and cut off from all central control, atropin will cause dilation, physostigmin contrac- tion of the pupil. The explanation of this is difficult, if we suppose that two antagonistic muscles are at work in the eye, for we would expect the poison to act on each alike, and that the pupil would remain unchanged in size. On the other hand, if there is only one muscle at work, we would say that atropin paralyses it, while physostigmin excites it to continuous and prolonged activity. The varia- tion in size of the pupil from emotion, obstructed respira- tion, and the like, is, on the other hand, of a central kind — that is to say, in such conditions the activity of the central nervous system is augmented or diminished I ii4 Physiology of the Senses with a corresponding effect upon the innervation of the eyes. Trie observation has been made that the pupil of the eye of a cat isolated after death, and with even the posterior segment of the eye cut off, will slowly contract on continued exposure to light. This appears to indicate that the iris is susceptible to the action of light even without the presence of a nervous mechanism. IL— PHYSIOLOGY OF VISION The optic nerves are the nerves of vision. When stimu- lated or injured no pain is caused, but only a luminous sensation is aroused. Nor are the nerve fibres sensible to light, except in and through the retina. Light falling upon the exposed optic nerve will cause no sensation, but if the nerve be now affected by mechanical, electrical, or chemical means, a sensation of a flash of light is ex- perienced. The sensation, however, is one of mere luminosity ; it is not accompanied by the perception of any object. In order that an object may be perceived, an image of it must be formed on the retina, and hence we note the double function of the eye, the power of responding to light, due to the structure of the retina, and the power of perceiving objects due to the nature of the transparent media in front of the retina. In many of the lower forms of animals we find nerves ending in coloured spots in the skin, and through these it may be the animal experiences a sensation of a special kind of light ; but, in the absence of a lens or other refractive media, images cannot be formed on these spots, and such animals can have no visual perception of external objects. It will conduce, therefore, to a clear understanding of this T/ie Sense of Sight 115 matter, if we consider briefly the nature of the stimulus — light — and the laws of its transmission through various media, that is to say, the laws of dioptrics. i. — LAWS OF DIOPTRICS The Physical Nature of Light. — It was once held that a luminous body shoots out from itself minute particles, which, passing to the observer's eye, give rise upon impact to the sensation of light. This corpuscular theory has now been entirely disproved, and it is generally held by physicists 'that the undulatory theory, first enunciated by Thomas Young, affords a satisfactory explanation of all the pheno- mena of light. According to this view, light, objectively con- sidered, is simply a mode of motion of a substance called the luminiferous ether which pervades, not only what is commonly regarded as space, but also all translucent sub- stances. By the molecular movements of luminous bodies this ether is set vibrating in series of waves. The com- ponent particles of these waves may be conceived to move at right angles to the direction of the ray of light, just as waves rise and fall while spreading outwards when the sur- face of calm water has been agitated by a stone. Thus a cork floating on the water, traversed by a wave, oscillates up and down nearly at right angles to the direction of the wave. These wave-like movements of the ether impinging on the retina set up in it changes which result in the sensation of light, but the sensation in no way resembles its physical cause, although it varies with variation of the stimulus. The intensity of the sensation varies with the amplitude of the waves. Large waves give rise to a sensa- tion of bright light, small waves to a sensation of dim light. Again, the sensation of colour depends upon the rapidity with which the waves follow one another. This rapidity, n6 Physiology of the Senses though inconceivably great, may still be accurately deter- mined. Ordinary sunlight, as Newton showed, is composed of a series of colours blended together, but yet separable one from another, because each colour is due to a series of waves differing in rate of succession from the others. Thus the waves of red light follow each other at the rate of about 435 millions of million times per second, while those of violet light succeed each other at about 764 millions of million times per second. Between these, we have an infinite number of series of waves, each giving rise to a special colour sensation, and so between the red and the -violet of the spectrum we have a gradation of colour roughly described as orange, green, blue, and indigo, but each of these is itself made up of countless shades, which melt as gradually and imperceptibly into one another as the colours in a sunset sky. The eye is not sensitive to vibrations of the ether succeeding each other more slowly than those of red light, although it may be demonstrated that these exist and originate electrical and thermal phenomena ; nor to those which come more quickly, although these have marked chemical activity, and give rise to fluorescence. Reflection and Refraction. — Light waves are propa- gated through the ether at about 190,000 miles per second, but the rate varies according to the medium through which the light is passing. When the medium is homogeneous the ray passes in a straight line. When it meets a polished surface it is reflected ; and the angle which the reflected ray makes with a perpendicular to the surface is equal to that which the ray meeting the surface, or, as it is called, the incident ray, makes with the same perpendicular. Further, the incident ray, the perpendicular, and the reflected ray will all be in the same plane. Few surfaces, however, are so highly polished as to conform entirely to the above laws. A certain part of the ray is usually The Sense of Sight 117 irregularly reflected or scattered, and it is owing to this fact that objects become visible, for it can be easily under- stood that if the rays were reflected entirely to the eye we would only be aware of the luminous body, and not of that which reflects the light. When a ray of light passing through one transparent medium, such as air, meets another, such as water, per- pendicularly, part of it is reflected upon itself, and part passes on in the same straight line through the water. If, on the other hand, the ray meets the surface of the water obliquely, the part which passes through the water continues in the same plane as before, but no longer passes in the same straight line. It is bent or refracted ^ out of its course. Some crystals have a power of dottble refraction — that is to say, the ray of light — entering them is broken into two rays. . 3 ' FIG. 45.— Diagram illus- each of which is deflected from the original trating the law of the course; but as in explaining the pheno- reflection of light from a plane surface. zO, mena Of vision we do not have tO deal Incident ray ; Or, re- with such substances, let it be understood flected ray- that what we have to say with regard to refraction refers merely to simple refraction or bending of the ray. The laws for single refraction have been thus stated1 — 1. Whatever the obliquity of the incident ray, the ratio which the sine of the incident angle bears to the sine of the angle of refraction is constant for the same two media but varies with different media. 2. The incident ray and the refracted ray are in the same plane, which is perpendicular to the surface separating the media. This ratio of the sines of the incident and refractive angles is known as the index of refraction ; and if the ray 1 Ganot's Physics, p. 466. n8 Physiology of the Senses be supposed to pass from a vacuum through any transparent substance, this ratio is known as the principal index of refraction for that substance, and is commonly represented by the letter /x. Knowing the index of refraction for any two media, we can calculate the direction which the ray of light will take as it passes through them. Each singly refractive substance, then, has always the same bending power due to its special elasticity and con- sequent interference with the velocity of the ray of light. Water interferes more than air, glass than water ; the diamond bends the ray of light more than any other known substance, or, in other words, is the most refractive sub- stance known. Effect of refraction on a ray passing through glass with parallel surfaces. — Suppose the ray EF (Fig. 46) passing „, through air meets obliquely the upper surface AB of a plate of glass hav- T^- NV >/ ing parallel surfaces. Part of the light will be reflected in the direction FK, part will pass through the plate, but not in the original direction FL ; TT it will be bent towards XY, the per- pendicular to the surface, and will FIG. 46. — Diagram illustrating , , , -„-, ,, . , the refraction of a ray of take the Path FG« Meeting the SUr- light. For description, see face CD, it nOW paSSCS OUt into the air, where it immediately regains its former velocity, or in other words, is bent back again to its former direction, so that it now emerges as GH, not indeed in the same straight line as before, but in a parallel direc- tion to its former course. Effect of refraction when light passes from air through a prism. — When light falls obliquely on the sides of a prism it is doubly bent, as may be seen from the accompanying C G The Sense of SigJit 119 figure. The ray GH (Fig. 47) meeting the surface AB at H, is bent towards DE in the direction HK, and emerging through the surface AC is bent away from EF in the direction D KL, that is to say, it is bent away from its original course, and deflected towards the base of the prism. B C FIG. 47. — Diagram illustrating re- D The amount of deflection de- ctive power of a prism' pends upon the shape and material of the prism, and on the angle at which the ray of light impinges on its surface. Action of Lenses. — A similar deflecting action is exer- cised by lenses, which may be looked upon as resembling two prisms in apposition by their bases or edges. Thus in Fig. 48, A and B represent pairs of prisms set respec- tively base to base, and edge to edge ; C, D, and E are convex lenses, or, in other words, are thicker at their FIG. 48.— Diagram showing comparison centre than at their circum- of lenses to prisms set base to base fgrenCC, and WOUld exercise a or edge to edge. C, Biconvex ; D, plano-convex ; E, concavo-convex ; deflecting power Upon rays similar to that of A ; and H are concave F, biconcave ; G, plano-concave ; H, convexo-concave lens. F, G, lenses, being thinner at their centres than their circum- ference, and would deflect rays of light in the same way as B. The biconvex lens is of most interest for our present purpose, for, like the transparent media of the eye, it has the property of condensing or focussing rays of light. The common burning-glass or biconvex lens has, as a rule, spherical surfaces. If AB (Fig. 49) represent a biconvex lens, and the line CF its principal axis, i.e. the straight line I2O Physiology of the Senses through the centre of curvature of its two surfaces, all rays parallel to CF meeting the surface ADB, will be brought to a focus at very nearly the point F, which is called the principal focus ; and, con- versely, rays spreading from F will pass through the lens, and emerge in a parallel direction. If " diver€ fr°m * F,G. 49-Diagram illustrating course taken . by parallel rays of light refracted by bicon- point f (Fig. 50) in the vexlens> axis of the lens outside of the principal focus, they will be brought to a focus at a point f on the other side of the lens known as its conjugate focus. FIG. 50.— Diagram illustrating the law of conjugate foci. If, as in Fig. 51, the rays diverged from /to the inside of F, they would still diverge on the other side of the lens ; FIG. 51. — Diagram illustrating position of virtual focus. but now if produced backwards, would form a virtual focus at/'. Formation of Images by Biconvex Lenses. — Any ob- ject at which we look may be regarded as made up of an aggregation of points, each of which sends a pencil of rays of light to the eye, and the main value of the lens for purposes of vision is its power of forming images of objects The Sense of SigJit 121 by combining again the scattered rays. Thus all the rays from A falling on CD (Fig. 52) may be collected at the point A', all the rays from B at B', and rays from all intervening points of AB will meet at points along the line A'B', and thus an image of AB is formed, but upside down or inverted. The size and position of the image depend on the position of the object with regard to the principal focus of the lens, and can be calculated by simple mathematical formulas. In Fig. 52, for example, the rays from the point A of the object AB may be supposed to be brought to a focus by the lens CD at the point A'. Those from B FIG. 52. — Formation of an image by a biconvex lens. at B', and all intermediate points in AB, at corresponding points in A'B'. We are now in a position to understand why a lens is required for vision. Were light simply to pass through the pupil and fall on the retina without refraction, from each point in the field of vision a cone or pencil of rays would pass to the retina and form a circle of light upon it, and these circles overlapping one another, as in Fig. 53, would simply give a sense of diffused light, and not the perception of each point separate one from another. But suppose the pupil were narrowed to the finest point, so that only one ray of light would pass in from each point of the object, as in 122 PJiysiology of the Senses Fig. 54, the amount of light admitted would be so infinitesimally small as to be unable to affect the retina. In avoiding overlapping, the amount of light admitted has become infinitely little ; or, in other words, as the pupil diminished in size FIG. 53.— Diagram showing overlapping of rays the object Would appear in the absence of a lens. dimmer and dimmer, until it ceased to be seen altogether, for the amount of the stimulus would be too small to excite the sensation of vision. But the refractive media of the eye acting like a lens con- dense the rays which have entered the pupil so as to form an image which, in the normal eye, falls upon the retina ; and each point of the image, being the focus or meeting- point of a vast number of rays coming from the correspond- ing point of the object, is sufficiently bright to stimu- late the retina to action. We may easily prove that such is the case. If an eye removed from itS FIG. 54- -For explanation, see text. socket be stripped posteriorly of the sclerotic coat, an inverted image of the field of view will be seen on the retina ; but if the lens or other part of the refractive media, be removed, the image will become blurred or disappear altogether. There are, however, two defects in ordinary spherical lenses which, as they affect the eye, deserve our notice. Spherical Aberration. — Any one who has attempted with a burning-glass to focus the rays of the sun upon a sheet of paper must have noticed that the circle of light, at first large and dim, gets smaller and brighter for a time The Sense of Sight 123 and then enlarges again, but the image of the sun thus formed is never reduced to a mathematical point. This is due to what is called the spherical aberration of the lens, and a glance at Fig. 55 will enable us to understand it. The ray of light CD, which passes through the centre of the lens AB, in Fig. 55, is not refracted at all, but passes on in a straight line. Rays near CD, such as E, E, are slightly bent and intersect CD at a considerable distance from the lens. Rays meeting the surface of the lens at points nearer its circumference than E, E, such as G, G, or K, K, are more refracted, and intersect CD at points nearer the lens. Thus, as we pass towards the circumfer- ence, the rays are more and more refracted, and do not B FIG. 55. — Spherical aberration. meet all at one point. Accordingly, when we interpose a screen in the path of the rays, while a few may be accurately brought to a focus upon the screen, the great majority are either still converging or now diverging, and they form concentric rings of light which blend with one another, or diffusion circles, as they are sometimes called, and these blur the image formed by the accurately focussed rays. By interposing a diaphragm, with a central aperture, the outer rays may be cut off and only those rays which pass near the centre will be brought to a focus, and thus the image will be made sharper. If the central part of the lens be more refrangible than the circumference, a similar 124 Physiology of the Senses result will be obtained, for rays passing through the former will be more refracted, and thus be brought to a focus nearer those that have passed through the circumference. Such a provision as this exists in the human eye, the centre of the crystalline lens being more refrangible than the outer parts. Chromatic Aberration. — The other defect in ordinary simple lenses is that when sunlight passes through them, owing to the different refrangibilities of the various coloured rays which go to make up white light, the sun's ray is broken up into its component parts, and some of these are bent more than others. This separation of the coloured rays is known as dispersion. The red rays being least FIG. 56. — Chromatic aberration. refrangible are less refracted than the orange, the orange than the yellow, and so on, the violet rays being most refracted of all. Thus, if rays pass through the lens AB (Fig. 56), we may suppose the red rays to intersect the main axis at R, the violet at V. If a screen be interposed in the position aa, there will be a coloured circular spectrum having the red to the outside and the violet to the inside ; but if the screen be placed at bb, the violet rays will now be outermost and the red rays to the inside. It was formerly supposed that the dispersive power of all bodies was alike, but it is now known that this is not so ; and by combining lenses of opposing action it has been found * possible to do away, to a very great extent, with the disper- The Sense of Sight 125 sion of the light, although it is still refracted. Such a lens is usually composed of a concave flint-glass (A, Fig. 57), and a biconvex crown-glass lens (B, Fig. 57), and is said to be achromatic, or in other words, not colour-producing. Optical Properties of a System of Lenses. T- . r .. , . - FIG. 57.— Achro- — If the rays of light emanating from an matic lens. A, object pass through a series of lenses, differ- Plano-concave lens of flint- ing in shape and refractive power, but having glass . B> bi_ their centres in one axis, the position and convex lens of size of the resulting image might be found by crown-glass- calculating and combining the effect of each lens in turn. This would, however, frequently lead to very elaborate cal- culations, and the researches of Gauss, Mcebius, Listing, and others have shown that for any system of centred spherical surfaces there exist six points known as cardinal points, through four of which pass planes perpendicular to the axis, and that if the position of these has been determined the direction of all rays of light through the system may be readily traced. The cardinal points are the first and second focal, first and second principal, and first and second nodal points, and the planes pass through the two first pairs. 1. The first focal point is so placed with regard to the system that all rays passing from it through the system, eme.'ge in a direction parallel to the axis of the system, while all rays parallel to the axis before entering the system are, having passed through it, gathered at the second principal focal point. This also holds good for all points in the planes through the foci perpendicular to the axis. 2. The first and second principal points are so situated that in the planes passing through them perpendicular to the axis — the principal planes — there are correspondent 126 Physiology of the Senses points on the same side of, and at the same distance from, the principal axis of the system, through which the refracted rays must pass. Thus each principal plane is the optical image of the other. 3. The first and second nodal points are such that all rays which before being refracted pass through one of them, seem after refraction to emerge from the other and in a direction parallel to what they had at first. 4. The first principal focal length is the distance between the first focal point and the first principal point. • 5. The second principal focal length is the distance between the second focal point and the second principal point. 6. The principal points are at the same distance from FIG. 58. — Diagram illustrating course of ray through a dioptric system. each other as the nodal points, and the distance between the first focus and the first nodal point is equal to that between the second focus and the second principal point. Then the distance between the first principal and first nodal points equals the difference between the first and second principal focal lengths. Given the cardinal points we may, then, trace the course of a ray through the system or calculate the position and size of the image of an object. Suppose in Fig. 58 FjF^ PjP^ NXN2 represent re- spectively the first and second focal, principal, and nodal points. Any ray AB from the first focal plane incident upon the first principal plane passes parallel to the main axis to The Sense of Sight 127 C, and thence in a direction parallel to AN, the line joining A to the first nodal point. To find the position of the image of any point A, we must trace the course of at least two rays from the point through the system till they meet. Thus, in Fig. 59, with the same FIG. 59. — Image of a point. letters as above, the ray AB parallel to the main axis passes through C, and thence through the second focus F2, while from N2 emerges a ray parallel to ANj which meets CF2 produced, at Ar 2. — THE DIOPTRIC SYSTEM OF THE EYE It was stated (p. 109) that light, before falling on the retina, passes through a series of transparent refractive sub- stances, viz. the cornea, aqueous humour, crystalline lens, and vitreous humour, and, with certain exceptions, which will be pointed out later, the eye may practically be con- sidered as composed of a centred system, composed of a convex refractive surface, the cornea, and of a biconvex lens, the crystalline lens. The cornea in reality has a double surface, but the outer and inner surfaces are so nearly parallel that the two may be regarded as one ; and although the lens differs much in the refrangibility of its different parts, its action as a whole may be taken as that of a homogeneous substance. The surface which exercises the greatest refractive influence is the anterior surface of the cornea, since the refractive powers of air and the 128 Physiology of the Senses substance of the cornea differ in a marked degree. On the other hand, the aqueous humour approximates so nearly in re- fractive power to the substance of the cornea that the refrac- tion in it may be neglected ; and, again, the refractive power of the vitreous is the same as that of the aqueous humour. Many careful investigations have been made as to the form of the various refracting surfaces of the eye, their relative distances from one another, and of the refractive powers of the different media concerned, and while it is found that the eyes of different persons, and even of the same person, differ to a considerable extent in all these respects, yet certain measurements have been obtained which may be regarded as representing those of an average normal eye. These being known, we can determine the position of the cardinal points, and thus calculate the course of rays of light in the eye. The following figures represent the latest and most accurate determinations : x — Index of refraction of the air . . . . . » = i. Index of refraction of the aqueous humour and vitreous body «'= 1-3365. Total index of refraction of the crystalline . .«"= 1-437 1. Radius of curvature of the cornea .... ?* = 7«829mm. Radius of the anterior surface of the crystalline lens r1 = lomm. Radius of the posterior surface of the crystalline lens /' = 6mm. Distance from the anterior surface of the cornea to the anterior surface of the crystalline = 3-6mm. Distance from the anterior surface of the cornea to the posterior surface of the crystalline . . = 7 -2mm. Hence, thickness of the crystalline . . . . e = 3.6mm. From these data, the following results have been calculated : — (A) Focal Points. I. Surface of cornea. First focal distance /0' = -L- = _Z^i_ = 23. 266mm. n'-i 1-3365-1 1 Landolt, The Refraction and Accommodation of the Eye, p. 79. The Sense of Sight 129 Second focal distance/0"= J^L = r'3365 * 7'^9 = ri-i I-3365-I II. Anterior surface of crystalline. First foca, distance/^-^ .^fj Second focaldistance/i"= - = -±-= ,42.853mm. III. Posterior surface of the crystalline. First focal distance /,'= ^_/ = _I^371x^_ = 8 n'-n" 1-3365-1-4371 Secondfocal distance// = -^-fl= — I-3365x6 = 79.7 r I3mm. n'-n" I-3365-I-437I (B) Principal Points. I. The principal points of the cornea coincide with its summit. II. The first and second principal points of the crystalline are at a distance of 2- 125970101. and 1-27560101. respectively from the anterior and posterior surfaces of the lens. (C) The Nodal Points of the crystalline coincide with its principal points. Hence it is deduced that — (1) The first principal focus of the eye is situated 13 -7451 mm. in front of the cornea. The remaining cardinal points of the eye are behind the cornea, and measuring from its anterior surfaces lie at the following distances. (2) The second principal focus of the eye is situated 22-82370101. behind the cornea. This distance, in other words, is the length in the normal eye between the cornea and the retina. (3) The first principal point, 1-75320101. ""(4) The second principal point, 2-iioimm. (5) The first nodal point, 6-96850101. (6) The second nodal point, 7-32540101. From Fig. 60 (p. 130) it will be seen that the prin- cipal points lie in the anterior chamber, the first nocjaj 1 3o Physiology of the Senses point in the lens, the second nodal point slightly behind it, the first principal focus in front of the eye, and the second principal focus at the posterior surface of the retina. The diagram represents what has been called by Listing the schematic eye. By its aid we may easily trace the course of all rays of light entering the eye. The principal points and the nodal points are seen to be respectively very near each other, and if each pair be FIG. 60. — Schematic eye. A, Anterior surface of cornea ; t/r', \}i", first and second principal focus ; H', H", first and second principal points ; K.', K", first and second nodal points ; F.c.,fovea centralis of yellow spot. (Landolt.) regarded as combined into one point, we simplify the con- ception of the eye very much, reducing it to a system having a single spherical surface separating the air from the more refractive media of the eye behind. The prin- cipal point is then at the surface, and the nodal point at the centre of the sphere, the focal points being situated as before. Such a conception is known as the reduced eye of Listing. The Sense of Sight 131 3. — ANOMALIES IN THE EYE AS AN OPTICAL INSTRUMENT While we may then form a conception of a mathematically correct eye, it must be borne in mind that all eyes present certain variations from the ideal form. 1. Thus the various refractive surfaces are not, as a rule, centred so that the optic axis or line joining their centres coincides with the line of vision, that is to say, with the line from the point viewed to the fovea centralis of the retina. The angle of the one axis to the other, where they meet at the nodal point, may be as great as 12°. This divergence of the optic from the visual axis is represented in Fig. 60, where it will be noted that the posterior end of the optic axis does not go to the fovea centralis. 2. Again, the centre around which the eye rotates is usually in the optic and not the visual axis, and, con- sequently, the line joining the point viewed with the centre of rotation of the eye, or, as it is called, the line of regard, does not usually coincide with the line of -vision. 3. Further, we have seen (p. 1 24) that in ordinary lenses, white light is broken up into coloured rays which are not focussed at The same point, and we saw how we can correct this by combining lenses of different forms and dispersive powers. Similarly, in the eye, the rays of light are broken up into their constituent colours, but this is done only to a very slight extent, and does not interfere with ordinary vision. In fact, its existence can only be determined by careful experimentation. When we look at red letters on a violet ground, the eye is soon fatigued by the effort to focus both colours on the retina at once, and we experience an unpleasant jarring effect ; or in looking at a violet flame which gives forth red and blue rays, we I32 Physiology of the Senses may either see a red flame with a blue halo, or a blue flame with a red halo, according as the eye is accommodated for red or blue. This may be called the defect of chromatic aberration. 4. The blurring of the image caused by spherical aberration (p. 122) is almost entirely corrected in the eye by the varying refractive powers of the media, especially of the C FIG. 61. — Astigmatism. The lens ACDEF has greater refractive power in the plane ACD than in the plane AEF ; rays in the vertical plane ACD will be brought to a focus at the point G, while those in the horizontal plane AEF are still converging to meet at the point B. If a screen be held at the point G, a horizontal line of light aa! will be seen ; if at the point B, a perpendicular . line be ; and if at intermediate points, ellipses of varying shapes as above. lens, by the influence of the iris in cutting off the outer rays, and by the shape of the refracting surfaces, which are not spherical, but of forms known as ellipsoids of revolution, that is to say, surfaces formed by the rotation of an ellipse upon one of its axes. 5. Astigmatism. — But these surfaces, while better adapted for vision than spherical surfaces, are themselves The Sense of Sight 133 usually somewhat irregular in this respect, that their curva- tures vary in different planes. In the vertical meridian the curve is in most eyes more convex than that in the horizontal ; and, as a result, rays in a vertical plane are brought to a focus nearer than those passing through the horizontal. Thus all rays diverging from a point cannot be exactly recombined to a point after passing through the eye, and a line is seen either in a horizontal or vertical direc- tion according to the position of the retina, or there is a diffusion ellipse for intermediate positions. Hence the name astigmatism given by Whewell, from a, without, and stigma, a point. That most eyes are more or less astigmatic is shown by the fact that to almost every man the fixed stars seem to twinkle or send out scintillations radiating from a centre. Were our eyes perfect, the stars would appear as luminous points, not FlG- 62.-Cylindrical lens to cor- ,., . .. . rect astigmatism in the eye. " star-shaped." Similarly, in look- Rays in two horizontal planes ing at the bars of a window, the are brought to a focus, but do not approximate in a vertical astigmatic eye cannot see both direction, vertical and horizontal bars at the same time with the same distinctness, one or other must be blurred by diffusion circles. Astigmatism may be regular, as above described, or irregular, the latter more especially being due to irregularities of the lens, while the former arises most commonly from the shape of the cornea. The effect is so slight in most eyes as to go unobserved, but it may be so great as to fequire the use of a lens consisting of the longitudinal segment of a cylinder, in which the convexity is greater in one plane than in another to compensate for the deficient convexity of curvature in one meridian as compared with the other (Fig. 62). 134 Physiology of the Senses 4. — ADJUSTMENT OF THE EYE FOR DIFFERENT DISTANCES When parallel rays, such as come, for example, from a star, fall upon the normal eye in a state of rest they are brought to a focus on the retina. If, however, the rays emanate from a point within a distance of about 65 metres (71 yards), they are sensibly divergent, and can only be brought to a focus upon the retina by an effort, and the nearer the object viewed is to the eye the greater must be the effort, until at last the eye becomes unable to gather the rays to a point at the retina, and the object is no longer distinctly seen. If, shutting one eye, we hold up a pencil in line with an object at some distance it will be found that both cannot be seen distinctly at the same time. If we see the distant object FIG. 63. — For description, see text. distinctly the outline of the pencil is blurred, and vice versa. The eye has the power of adjusting itself so that all rays from beyond a certain near point may be focussed on the retina. Thus if the rays from a point p (Fig. 63) are re- fracted so to meet at r the retina,/ will be seen distinctly, but if the point p be now moved to the point /', unless the eye be adjusted for the change, the rays from p' will be focussed behind the retina, and the point p would be seen . indis- tinctly. Now, there are two ways in which this adjustment might be effected. The length of the eye might be varied to meet the varying distance of the focal' point, just as a photographer moves the sensitive plate of his camera back- wards or forwards to bring it into focus. But, as a matter The Sense of Sight '35 of fact, another process takes place in the eye. The retina is not moved backwards or forwards, but the refractive power of the crystalline lens is changed by an alteration of its thickness. The more curved the surfaces of a lens are, the greater is its refractive power. Now, when we look at distant objects, and no effort at accommodation is required, the anterior surface of the lens is kept flattened by the pressure of its capsule and by the elastic pull upon it of the anterior suspensory ligament — an elastic pull which involves no muscular strain, and consequently no fatigue. But when we wish to look at a near object, the ciliary muscle (see p. 101) contracting" pulls forward the suspensory ligament FIG. 64. — Mechanism of accommodation. A, The lens during accommodation with its anterior surface advanced ; B, the lens at rest ; C, position of the ciliary muscle ; D, the vitreous humour ; a, the anterior elastic lamina of cornea ; c, corneal substance proper ; 6, posterior elastic lamina. and diminishes its circle of attachment, its tension is lessened, the pull on the capsule of the lens diminishes, and the lens, by its own elasticity, assumes a more spherical shape, its anterior surface moving forward, and its power of converging rays being increased. The nearer the object the greater the effort required, and when long sustained the greater is the fatigue experienced. As a rule, however, we are unconscious of the effort, although, as will be seen, the feeling gives us valuable aid in judgment as to the distances of objects. The accompanying diagram (Fig. 64) repre- sents the change, the right side B showing the condition of rest, the left A the state when the eye is adjusted for near Physiology of the Senses FIG. 65. — Reflected images in the eye. A, for distant B, for near vision. sight. The change in the curvature of the anterior surface of the lens may be demonstrated as follows : Let the observer in a dark room, looking at the side of the eye to be examined, note the reflections of a candle flame held to the other side, and in front of the eye observed. Two bright points can be readily seen — one the reflection of the flame from the surface of the cornea, and one from the anterior surface of the lens — and, with care, a third, much fainter, from the posterior surface of the lens. When the person whose eye is being examined is directed to look as at an object at a great distance, the three points of light will have the position shown in A (Fig. 65) ; and now on adjusting the eye so as to see an object close at hand the middle point of light moves forward, nearer to the corneal reflection, and becomes smaller as in B. This is due to the bulging forward of the lens, and the consequent reflection of the light from a surface nearer the cornea, and more curved than before. The experiment can be readily performed in daylight by means of the phakoscope invented by von Helmholtz, which IG. 66- — Phakoscope. The ob- server looking through the aper- ture a sees images of the slits bb' reflected from the observed eye situated at the distant side of the phakoscope, and accom- modated first for distance, and second for near vision, the re- gard in the latter case being fixed on the needle-point in the window c. The Sense of Sight 137 consists of a darkened box applied to the eye, with aper- tures at convenient positions for the light, for the eyes of the experimenter and of the person observed, and with an opening through which the eye to be observed may look. Careful measurements of the sizes of the reflected images have shown that the image on the anterior surface of the lens becomes smaller when we look at a near object, another proof that the lens becomes more convex anteriorly. There is also a slight increase in the posterior convexity of the lens. The Near Point of Vision. — The range of accommoda- tion is limited. It begins for objects at about 65 metres (71 yards) from the eye, and for normal eyes reaches to x y FIG. 67. — Scheiner's experiment. For description, see text. within 20 centimetres (8 inches). The position of the near point of any eye may be readily determined by the classical experiment of Scheiner. It is performed as follows : In a thick card make two small holes with a needle at a distance not greater than the diameter of the pupil, and holding the paper closely to the eye look at the needle through the holes. If the needle be held 4 or 5 inches from the eye two points will be seen, but as the needle is gradually moved farther away the two points will be seen to coalesce into one point, and they do so at the near point of vision, namely, 8 inches from the eye. The meaning of this will be understood from the diagram in Fig. 67. If the needle is at the nearest point at which 138 Physiology of the Senses the rays coming from it to all parts of the pupil can be collected to one point on the retina, the cones of rays passing through the apertures will be collected at r, and we see the needle single, but on bringing the needle nearer to the eye we are unable to adjust the eye for the divergent rays, and it is as if the retina were situated at zz, and two points a and b will be seen ; but as these are due to circles of diffusion and not to rays brought to a point, the image on the retina is blurred, and not so bright as before, owing to the lessened quantity of light admitted by the single hole. As the image is projected outward through the nodal point N, the image of b will be seen in the line bb', and that of a in the line aa't in other words, the real point seems to be split into two, one on each side of the true position. The distances given above for the far and near points are those for a normal eye at rest, in which the optic axis is of such a length that parallel rays are brought to a focus on the yellow spot (Fig. 68, i). Such an eye is called emme- tropic, or an eye in measure. But many eyes are not so adapted ; they have the retina either before or behind the focal point, and are then said to be ametropic, or not in measure. The axis may be too long, and parallel rays are focussed before they reach the retina (Fig. 68, 4), as in the short-sighted, myopic, or hypometropic eye ; or the axis may be too short, as in the long-sighted or hypermetropic eye, and the rays are brought to a focus behind the retina (Fig. 68, 3). A short-sighted person, who desires to see distant objects, wears spectacles with concave lenses to make the parallel rays diverge, so that on passing through the eye they will be brought to a focus farther back than usual, and so upon the retina ; while in viewing near objects, as in reading, the book is held nearer the eyes to give greater divergence to the rays. The long-sighted person, on the other hand, The Sense of Sight 139 wears convex lenses, so that the rays may be brought more quickly to a focus, and in reading he holds the book at FIG. 68. — i, Emmetropic eye ; 2, normal eye accommodated for near vision by increased curvature of the anterior surface of the lens ; 3, hypermetropic eye ; 4, myopic eye. arm's length for a similar reason. Further, an eye of normal length may gradually lose its power of adjustment for near objects, a condition common in old age, and we have I40 Physiology of the Senses what is known as the presbyopic eye. In the eye of an old person the parts are deficient in elasticity, and the fibres of the ciliary muscle are probably less powerful than in early life. The anterior surface of the lens cannot therefore become sufficiently convex for objects viewed a little beyond the near point of distinct vision. In other words, the near point in a presbyopic eye is farther back than normal, and hence, in reading, the head is thrown back and the news- paper held as for away as possible. In this case, too, convex lenses are used to compensate for the lost power of adjustment for near objects. Irradiation. — A minor result of defective power of FIG. 6>— Irradiation. accommodation is to be found in the phenomenon known as irradiation. When we look at a bright object on a dark ground it seems larger th?n when a dark object of similar size is seen on a light ground. People dressed in white look larger than when in black Note also the two small squares in Fig. 69. The white seems larger than the black, although they are of exactly the same size. This is probably due in part to the formation of circles of diffusion, the more powerful stimulus of the rays from the white surface annulling the less intense rays from the dark border. An interesting example of this is the effect produced on the eye by the glowing filament of the electric lamp. The The Sense of Sight 141 filament may form a loop, but this is not seen when the full light of the lamp meets the eye. We see only a brilliant light But if we cut off some of the rays by the intervention of a plate of smoked glass, or by winking the eyes rapidly, the filament is distinctly seen, although apparently broader than it really is on account of the intensity of its luminosity. Entoptic Phenomena. — In describing the effects of refraction on the rays passing through the eye, we have hitherto spoken as if the transmitting media were perfectly transparent in all parts. It has now to be observed that in almost every eye there are small opaque bodies which intercept the light as it enters, and throw shadows on the retina. These shadows projected out- wards give the impression of rounded or filamentous bodies floating hi space. They may be well observed by looking with half-shut eyes at a white cloud, when they will be seen floating away and eluding our efforts to keep them at rest. They have been called on this account musca voKtantes, and their fleeting character is due to the fact that they are not as a rule directly in the line of distinct vision, and hi our attempt to gain a direct view of them we move the eye and with it the substance which gives rise to the appearances. The opaque particles may be either in front of the retina or in the retina itself and one of the latter phenomena, namely, the shadows of the retinal vessels, is of especial interest, not only from its peculiar appearance, but also from the proof which it affords that the layer of rods and cones is the part of the retina sensitive to light. It may be studied as follows. In a dark room cast a bright ray of light sideways upon the cornea. This pene- trating to the retina forms there a luminous image which itself is reflected to other parts of the interior of the retina. Qne of these reflected rays may in its course impinge upon 142 Physiology of the Senses a retinal vessel which casts its shadow on the outer corre- sponding part of the retina. The part of the retina upon which the shadow falls, refers this outwards through the nodal point of the eye. The path described is traced in Fig. 70, A. The ray b passing to c' and reflected thence, falls on a vessel x in the retina, and a shadow is cast at d which is referred outwards in the direction da'. If now the source of light be moved to b' the ray will pass to <:, be reflected in the direction «/', and intercepted at x} FIG. 70. — Diagram to illustrate the formation of Purkinje's figures. with consequently a shadow on d' which is referred out- wards in the direction d'a. If the ray of light cannot enter the eye by the pupil, but merely passes through the sclerotic, we will have the result depicted in Fig. 70, B. A ray of light entering at a" is intercepted by a vessel and 3.65 /* = TOVi> of an inch. The Sense of Sight 147 subtend the angle of 50" it must appear as a point. The fixed stars we know to be vast suns, but they appear to us as mere points of light because their dis- tance is so great that they subtend a very small visual angle. Nay more, many stars long supposed to be single have, by the aid of powerful telescopes, been shown to image smaller be double, triple, quadruple, or even mul- than the dia- meter of a cone tiple stars, at vast distances from one another, may affectone, or and yet appearing as one to the naked more than one> cone at the same eye. time. The image For distinctness of vision the eye must affecting two cones is actually have what we may call resolvmg power, the smaller than that power of keeping each point of the image affecting one clear and distinct from its neighbour, and this power we have said is greatest in the yellow spot. For example, the two dots below are easily recognised as two, if we look directly at them ; but if we look a little to one side, the two will apparently fuse into one whenever their images are displaced from the yellow spot and fall upon an adjoining part of the retina. By means of a pencil we can map out on the page an area of irregularly oval shape corresponding to the oval shape of the yellow spot, an area in which the two dots are seen as double and not fused. The greater the distance between the dots, the further, cceteris paribus, from the yellow spot of the retina may they be distinguished as such, or in other words, the further we pass on the retina from the yellow spot the less resolving power does the retina possess. We have indicated above the shortest distance between two points which will allow of their being seen as two. A much smaller area of stimulation of the retina is sufficient 148 Physiology of the Senses to give rise to distinct vision. A luminous point or line may be seen as such which gives rise to an image that occupies only a very small part of a cone or row of cones. An object -04 mm. (^^ of an inch) in breadth at a distance of 25 mm. (i inch) from the eye gives a retinal image of about -002 mm. fa 2 \ 0 0 of an inch) in breadth, and yet it is distinctly visible. This is, however, by no means the minimum msibile. Objects as small as the 4 o 0*0 o o °f an mc^ m diameter (about one -tenth of the length of a wave of light) may be seen with the highest powers of the modern microscope. It is hardly necessary to state that even these minute objects are many thousands of times larger than the molecules or atoms of matter dealt with by the physicist. The Size of the Retinal Image. — The size of the image of an object upon the retina may be calculated by a simple formula if we know the size of the object, its distance from the nodal point, and that of the nodal point from the retina. In the average normal human eye the distance of the nodal point from the retina is approximately 16 mm., and from the nodal point to the anterior surface of the cornea 7 mm. Let the size of the object be represented by X, its distance in mm. from the anterior surface of the cornea by /, and therefore from the nodal point by p + 7. Then p + 7 : 1 6 : : X : x, the size of the image ; or x— — . Suppose, for example, the object looked at be the page of this book, which is nearly 182 mm. long, and that the book is held half a metre (500 mm.) from the eye. Then the length of the retinal image of the page will be i82x 16 *•=— — — =5*7 mm., or a little less than one quarter of an inch. Again the length of any small letter on the page is approximately I mm., hence the height of its The Sense of Sight 149 retinal image, the book being held as before, will be of an inch' The above-mentioned formula, however, gives only the length of any diameter of the object in a plane perpendicular to the line of vision. To calculate the area of the image on the retina we have only to remember that the area of the image is to the area of the visual field occupied by the object as the square of the distance of the image from the nodal point is to the square of the distance from the nodal point to the object. The flat retinal image cannot, of course, correspond in area to the superficial area of a solid body, but only to a part of the field of vision cut off by a plane projection of the object upon it. It is as if the visual field were a canvas, every point of which is filled by the representation of some external object, and the retinal image is an exact copy, but reduced in size, of nature's picture. The full moon and a ball held in the hand give alike a flat circular retinal image, but in the " mind's eye " each may be seen as a sphere, although the play of light and shade on the nearer object renders the effort of imagin- ation easier with it than in the case of the more remote. The Blind Spot. — It is interesting to note that near the area of greatest sensitivity to light we have a spot in the retina which is devoid of rods and cones, and hence is quite unaffected by images formed upon it. This is the optic papilla, or place of entrance of the optic nerve, and its diameter being about i«8 mm., it subtends a visual angle of about 6 degrees. Lines drawn from the border of the optic pore to the nodal point and produced outwards will enclose a flattened cone whose base is contained within the visual field, and within which all objects will be invisible to the unmoving eye. Suppose, for example, the left eye being shut, the right eye be fixed upon the cross in Fig. 74. 150 Physiology of the Senses When the book is held at arm's length, both cross and round spot will be visible ; but if the book be approximated to about 8 inches from the eye, the regard being kept steadily upon the cross, the round spot will at first dis- appear, but as the book is brought still nearer both cross and spot will again be seen. It may also be noted in this ex- periment, that there is no consciousness of a break of continuity in the visual field, no sensation as we might imagine there would be of darkness ; to put it generally, there being no stimulation, there is not consciousness of a lack, but a lack of consciousness. An attempt has been made to determine the rate of decrease of acuteness of vision as we pass outwards from the yellow spot, and Vdlkmann holds that it diminishes proportionally to the square of the distance from the yellow FIG. 74. spot, but the determination is, in its nature, very hard to make, and much depends on individual peculiarities. Action of Light on Retina. — This will be the more readily understood if we consider for a moment the intimate nature of the action of light on the retina. It has been experimentally observed that if the eye be kept in the dark for a time, and if light then be allowed to fall full on the retina, there is a change in its electrical condition. This phenomenon is evidence of change in the condition of the molecules of the sensitive parts of the retina, which might be merely a change of rate of molecular motion such as results from a variation of temperature of a body, or it might be due to a chemical transformation or rearrangement of the molecules so as to form new chemical substances. The Sense of Sight 151 That the latter is more probably the case may be held upon various grounds. If heat rays be substituted for light in the foregoing experiment the electrical change will not occur. Further, it has been observed in the frog's eye (the retina of which contains only rods, and which is also well adapted for the observation of the electrical change produced by light) that in the outer part of the rods of quiescent eyes there is a pigment of a purple colour derived from the pigmented layer outside of Jacob's membrane, and on exposure of the eye to ordinary light this purple changes to yellow and then to white. On removal of the light the pigment slowly reappears in the rods. This pigment is not found in the cones of the retina of other animals, and hence is absent in the yellow spot. As the yellow spot is the seat of acute vision in daylight we must infer that the purple pigment is not essential to vision, but we must not conclude from this that it has no visual function. For if we pass from darkness to bright light, the eye at first is dazzled until possibly the visual purple is bleached, or in other words, until the eye's sensibility to light is diminished ; and, on the other hand, if the eye has been exhausted by bright light we do not see objects well in a dim light until the visual purple is restored. In a dim light, the pupil of the eye is dilated, and rays affect the retina round the yellow spot. It would thus appear that visual purple assists vision in dim light while it is not necessary in bright light ; but as we have a chemical change in the purple pigment, so we may have in the yellow spot substances which undergo chemical change, although this be not manifest to the observer. The yellow spot is thus better adapted for acuteness of vision, for concentration of the attention upon minute detail, while the surrounding parts of the retina are more sensitive to the action of light and more fitted for observ- 152 Physiology of the Senses ing bodies emitting or reflecting but a small quantity of light. Amount of Light required to excite the Retina. — The smallest amount of light that will excite the retina cannot be stated, as so much depends upon the part of the eye affected, its state of vigour or exhaustion, its previous education, and the like. Thus the sailor will see land in the distance which is imperceptible to the landsman ; the Oriental will distinguish shades of colour more accurately than the European ; and the artist will differentiate where the untrained eye sees but one tint. Again, the exhausted eye will fail to see what is readily perceptible to the fresh eye of one newly wakened from sleep ; and the star, whose faint light is unseen by direct vision, may be seen when its ray meets the retina a little to the outside of the yellow spot. Nay more, even when we are enveloped by the deepest darkness, and when the eyes are shut, the ordinary field of vision seems still irradiated by a faint pervading glow, known as the specific light of the retina, which upon slight pressure by the hands may be broken up into a mosaic of fleeting patterns. The sensations thus excited by pressure are called phosgenes. The retinal light is caused by changes in the retina due to variations in the blood supply. Persistence of Retinal Impressions. — The substance of the retina is more or less affected according to the brilliancy of the light and the length of time during which it acts upon the eye. A feeble light acting for a short time will leave but a transient effect, while a strong light, such as that of the sun or of the electric spark acting for an instant only, may give rise to impressions lasting many minutes, or, if the exposure be prolonged, even to permanent damage to the eyesight. If we look directly at the sun and then turn our eyes to the ground, or towards a darkened clo?id, The Sense of Sight 153 the image of the sun formed upon the retina has been as it were so deeply graven, the retinal structure has been so changed, that for several moments we fail to see the object towards which the eyes are turned, and we see a round red spot, or several red spots, if the eyes were not steady when the sun was in view. This spot is a spectrum or after-image of the sun projected outwards upon the visual field, moving with every move- ment of the eye, and seen even when the eyes are closed. If a piece of burning wood be shaken rapidly to and fro, we see a line of light, because adjacent points on the retina are consecutively stimulated, and the fusion of the after-images gives the sensation of continuity. A disk with alternate lines or sectors of black and white radiating from the centre will, when rotated rapidly, seem to have a uni- form gray colour due to the fusion of the black and white FlG< ^ _The disk A having bkck and Spectra; but if seen by the white sectors, when rotated rapidly light Of the instantaneous S^es an even gray tint as in B. electric spark, each black and white line or sector will be visible because the time of illumination and consequent stimu- lation of the retina is so short that there is no time for the superposition of the images one upon the other. Similarly, if various simple colours be painted on the disk, their spectra will, on rotation of the disk, be fused together, giving rise to a sensation of the colour due to their combination. If a series of twenty or thirty instantaneous photographs be taken at short but equal intervals of time of an animal performing some movement, as, for example, a horse leaping over a gate, the pictures fixed to a disk will, when rotating quickly, seem to coalesce each with- its predecessor so as to give the impression of the 154 Physiology of the Senses horse in actual movement. This is the principle of the toy known as the Thaumatrope or Wheel of Life. Since the after-image in the instances above mentioned has an appearance similar to that of the object viewed, it is called a positive after-image. But there is another kind of after- image, the negative, which is due to a slightly different cause. Suppose we look fixedly at an object for thirty or forty seconds, so that the eye becomes fatigued, and then turn our eyes to a surface of uniform tint, we will see an image floating on the wall in which the lights will be reversed — what was dark will be light, what was bright will be dim. In this case the rays of light reflected from the wall have most effect upon those parts of the retina which are least exhausted, while those parts formerly much stimu- lated will now look dark, not being so easily excited to action. The persistence of retinal impressions is probably in part the cause of the phenomenon known as irradiation (see p. 140). The eye moving rapidly over the white surface, and being more affected by its light, the dark area seems the smaller. It may also be that there is a slight dispersion of light from the retinal elements directly affected to those immediately adjoining, which makes the image larger, and so leads to an erroneous judgment as to the size of the white object. A further and most interesting illustration of the per- sistence of the retinal state may be studied as follows : Look steadily for about half a minute at a disk with alternate black and white sectors which is being slowly rotated. Then turn the eyes to a sheet of paper upon which a number of dark spots may be seen. These will seem to rotate in a direction contrary to that in which the disk was turning. The effect here is of the same nature as the phenomenon often seen on the deck of a steamer. If we lean over the side of the vessel, and watch the water as The Sense of Sight 155 the vessel glides along, it soon seems as if the ship were stationary and the water near us in rapid motion in the direction opposite to that in which we are moving — the apparent rapidity gradually diminishing as we look at more remote parts of the water. If we now gaze at the deck, the part near us will seem to move towards the bow of the ship, the rest of the deck remaining fixed. Different parts of the retina have been stimulated by rays from different parts of the surface of the water apparently moving at different rates. But when the whole visual field is occupied by the deck, the various parts of which are fixed relatively to each other, the persistence of the retinal impression of greater movement in one part of the visual field than in the rest of it causes us to imagine that parts of the deck, which rela- tively to the rest of the deck are stationary, are actually in motion. 5. — SENSATION OF COLOUR In considering the physical nature of light (p. 1 1 5), we saw that the shade of colour, according to the most likely hypo- thesis, depends on the rate of vibration of the luminiferous ether, and that solar or white light is a compound of all the colours in definite proportion. A body which reflects solar light to the eye without changing this proportion appears to be white ; if it absorbs all the light so as to reflect no light to the eye, it appears to be black. If a body held between the eye and the sun transmits light unchanged and is transparent, it is colourless ; but if translucent, it is white. If it transmits or reflects some rays and absorbs others, it is coloured. If, for example, it absorbs all the rays of the solar spectrum but those which give rise to the sensation of greenness, we say that the body is green in colour. But this greenness can only be perceived if the rays of light falling on the body contain rays having the special 156 Physiology of the Senses vibratory rate that is required for this special colour. For, if we use as our light any other pure coloured ray of the spectrum, say the red, its rays being absorbed the body appears to us to be black. A white surface seen in a red light seems to be red, in a green light, green, as it reflects all colours alike, absorbing none. To the normal eye the colour depends, then, on the nature of the body and of the light falling upon it, and the sensation of colour only arises when the body reflects or transmits the special rays to the eye. If two rays of FIG. 76. — Lambert's method for studying combinations of colour. The rays, e.g., from the red wafer d reflected by the glass plate a to the eye E are pro- jected outwards and superposed on the blue wafer b, which appears of a rose colour. different colour affect one part of the retina at the same time, they are fused together, and we have the sensation of a third colour different from its cause. Thus, if red be removed from the solar spectrum, all the others com- bined will give a sensation of a greenish yellow, although we cannot, with the unaided eye, analyse this into its com- ponents. Fig. 76 shows a method by which different -coloured rays may be made to converge from two bodies on the same part of the retina. Von Helmholtz gives the follow- The Sense of Sight '57 ing table as the result of mixing the pure colours of the spectrum : — V. B. G. Y. R. R. Purple. Rose. Dull Yellow. Orange. Red. Y. Rose. White. Yellow Green. Yellow. G. Pale Blue. Blue Green. Green. Bl. Indigo. Blue. V. V. Thus a mixture of red and violet gives purple, of yellow and blue, white. Here we must guard against a possible error. The effect of say yellow and blue light acting at once on the eye is to cause a sensation of white light ; but if we mix blue and yellow pigments the mixture looks green, because the one pigment cuts off the rays at the red end, the other those at the violet end of the spectrum, and the only rays reflected are those of the green or middle part of the spectrum. In the one case we have a com- bination of colours, in the other each absorbs a part of the spectrum previously seen when the pigments were unmixed. Similarly, if the colours of the spectrum be painted upon 158 Physiology of the Senses a disk, in due proportion and in proper series, the disk will, when quickly rotated, look white. This is due to a fusion of colour effects, not to a mixture of the pigments. Complementary Colours. — When one colour is separated from the spectral series, the rest, as we have said, may be combined in the retina to give a sensation of one colour, and this colour will, if recom- bined with the one originally separated, give the sensa- tion of white light. These two colours, then, are said to be complementary to each other, and every colour in the spectrum may thus be said to be the complement of all the others. By combining colours at opposite ends of the spectrum, the effect of the intermediate colours may be produced ; but the lowest and highest of the series, the red and the violet, cannot be thus formed. They may be regarded, therefore, as primary colours — colours which cannot be produced by the fusion of others. If to red and violet we add the colour whose vibratory rate is about midway intermediate, viz. green^ we may, by their combination, give rise to a sensation approaching that of white light. Consequently these three colours have been designated the fundamental colours. Colour as dependent on the Retina. — Our per- ception of colour depends, however, not only on the physical stimulus of light, but also on the part of the retina affected. In and around the yellow spot where the cones are most numerous, the power of distinguishing shades of colour is greatest. Instead of seven colours in the spectrum more than two hundred different tints may be distinguished. Outside of this central area lies a middle zone in which much fewer tints are seen, these being con- fined, indeed, to shades of blue and yellow ; while in the front part of the retina all colour tints are lost, and objects give rise simply to the sensation of dark shadowy The Sense of Sight 159 bodies without colour. Moreover, the range of spectral colours varies with the individual. Colour Blindness. — Every colour has three qualities : 1 i ) hue, or tint, as when we speak of red, green, or violet ; (2) purity, or degree of saturation (due to a greater or less admixture with white), as when we designate a red or green as deep or pale ; and (3) brightness •, or intensity, or lumin- osity, as when we describe the tint of a red rose as dark or bright. On comparing two colours we say they are identical when they agree as to these three qualities. Observation has shown that in thus assorting colours, about ninety-six out of every hundred men will agree as to identity or difference of colour, and may be said to have normal colour vision, while the remaining four men will show a defective perception of colour, and are called colour blind. It is curious that colour blindness is about ten times less frequent in the female sex. This condition is congenital and incurable. It is due to some unknown peculiarity of the retina, or nerve centres, or both, and it is to be dis- tinguished from transient colour blindness, sometimes caused by the excessive use of tobacco and by disease. There is probably no such condition as absolute colour blindness, in the sense of total insensibility to colour ; a few rare cases have been noticed in which there was apparently only one colour sensation ; a few cases occur of failure to distinguish blue from green, and insensibility to violet is rare. The common form of defective colour vision is Daltonism or red-green blindness, of which there are two varieties — the red-blind and the green-blind. In each variety there are many gradations of sensibility. To the red-blind red appears as a dark green or greenish yellow, yellow and orange appear as dirty green, while green is green and brighter than the green of the yellow and orange. A green-blind person, on the other hand, would call red 1 60 Physiology of the Senses dark yellow, yellow would be yellow except a little lighter than the red he calls dark yellow, and green would be described as pale yellow. When asked to look through a spectroscope at the spectrum, the extreme or low red is absent to the red blind, and the brightest part of the spectrum appears to him to be the green, while to the normal eye and to the green-blind eye the spectrum is most luminous in the yellow. Seeing that green lights imply safety, and red lights danger, on our railways, and that in navigation a green or red light on the port or starboard side shows the course a vessel is taking, it is evident that no one who is red- or green-blind should be employed in the services, and accordingly various tests are now in use for the detection of such defects. The most efficient is the wool-test of Holm- gren, which consists of three skeins of wool dyed with standard test colours, namely, a light green, a pale purple or pink, and a bright red. Other skeins of reds, oranges, yellows, yellowish greens, pure greens, blue greens, violets, purples, pinks, browns, and grays, all called confusion colours, are provided, and the examinee is requested to select one and match it with one of the test colours. Suppose the light green skein is shown first If the examinee matches grays, brownish grays, yellows, orange, or faint pink with this, he is colour blind. Then he is shown the purple skein. If he matches with this blue or violet he is red- blind, but if he selects only gray or green he is green-blind. Finally, he may be shown the red skein, having a bright red colour, like the red flag used on railways. A red -blind person will then match with this green or shades of brown, which to a normal eye seem darker than red ; while if he be green-blind he will select shades of these colours which look lighter than red. Violet blindness is recognised by the examinee confusing red and orange with purple. The Sense of Sight 161 Coloured after-images. — The power of the retina in distinguishing colours depends also upon its freedom from fatigue. As there may be after-images of form, so there may be after-images of colour, and these after-images may be negative or positive. If positive, we see with the eyes shut the same colour as we have just been looking at ; if negative, we see the complementary colour, and as we continue examining it we find the colour changing and fading away, the lighter tints merging into the darker. The eye fatigued by gazing at a red square, will, when turned to a white surface, seem to see a bluish-green square on the white ground, for the fatigued eye responds more readily to the stimulus of the other colours of the spectrum ; and these give, when fused, the complementary colour -(p. 158). Similarly, a white square seen against a bluish-green background will have a reddish tint, probably because the eye moving quickly over the coloured field, and becoming thereby fatigued, responds more readily to the red rays in the white light than to its other component parts. This is known as the phenomenon of contrast. Theories of Colour Vision. — How comes it that we can perceive differences in colour ? This question has never been satisfactorily answered, because the changes caused in the retina by the action of light are too minute to allow of direct observation. Many hypotheses have been framed, but none of them meets all the requirements of the case. We may look for the cause in various direc- tions. We might suppose a molecular vibration to be set up in the nerve-endings synchronous with the undulations of the luminiferous ether, without any change in the chemical constitution of the sensory surface ; and we might suppose that where various series of waves corresponding to different colours act together, these are fused together, or interfere with each other in such a way as to give a M 162 Physiology of the Senses vibration of modified form or rate corresponding somehow to the sensation arising in consciousness. Or again, we might suppose that the effect of different - coloured rays is to promote or retard chemical changes in the sensory surface, which again so affect the sensory nerves as to give rise to differing states in the nerves and nerve centres with differing concomitant sensations. The former of these lines of thought guided Thomas Young, the great expounder of the B Red. H. G ^ F E D Violet. Indigo. Blue. Green. Orange yellow. FIG. 77.— Diagram to illustrate the Young-Helmholtz theory of colour vision. The lines with the letters B, C, D, etc., below the curves indicate certain fixed lines in the solar spectrum, whose wave-length has been determined. Take D, the height of the two curves above it indicates the degrees of stimu- lation of the two sensations red and green that produce orange-yellow. Again, at E we see a mixture of the three sensations that produce spectral green. (Report of the Committee of the Royal Society on Colour Vision.) undulatory . theory of light, in his attempt at explaining colour perception ; and his theory adopted and worked out by von Helmholtz has, been received with much favour. Het supposed that there are three fundamental colour sensa- tions— red, green, and violet — by the . combination of which all other colours may be formed, and that there are in the retina three kinds of nerve elements, each of which is specially responsive to the stimulus of one colour, and much The Sense of Sight 163 less so to the others. If a pure red colour alone act on the retina, only the corresponding nerve element for red sensa- tion would be excited, and so with green and violet. But suppose the colour be mixed, then the nerve elements will be set in action in proportion to the amount of constituent excitant rays in the colour. Thus, if all the nerve elements be set in action, we shall have white light ; if that corre- sponding to the red and green, the resultant sensation will be orange or yellow ; if mainly the green and violet, the sensation will' be blue or indigo, and the like. Von Helm- holtz succinctly puts it as follows : — (1) Red excites strongly the fibres sensitive to red, and feebly the other two — sensation, red. (2) Yellow excites moderately the fibres sensitive to red and green, feebly the violet— sensation, yellow. (3) Green excites" strongly the green, feebly the other two- sensation, green. (4) Blue excites moderately the fibres sensitive to green and violet, and feebly the red — sensation, blue. (5) Violet excites strongly the fibres sensitive .to violet, and feebly the other two — sensation, violet. (6) When the excitation is nearly equal for the three kinds of fibres, then the sensation is white. Another mode of expressing the theory is to say that each primary sensation of red, green, and violet is excited in some degree by almost every ray of the spectrum, but the maxima of excitation occur at different places, 'while the strength of stimulation in each case diminishes in both direction's from the maximum point, Thus, when the three sensations are equally excited, white light is the result ; green is caused by a very weak violet sensation, a stronger red, and a still stronger green sensation. At each end of the spectrum we have only the simple sensations of red and violet, and all the intermediate colour sensations, are compounds of varying proportions of the three primaries. 164 Physiology of the Senses According to this theory, red blindness is attributable to the absence of the red sensation, and green blindness to the absence of the green sensation. When the green and violet sensations are equal in amount, a red -blind person sees what is to him white, and when the red and violet are equal a green-blind person will have a sensation of what in turn is to him white, although to the normal eye these parts are bluish green in the one case and green in the other, as the green sensation is in each added to the sensations of red and blue. But while this theory explains certain phenomena of colour blindness, of after-images, and of colour contrast, it is yet open to serious objections. There is no proof, one way or other, of the existence of three kinds of nerve elements corresponding to the three fundamental colour sensations. Again, it does not explain how red should have to the colour-blind person a similar appearance to green, or how it should give rise to a sensation of colour at all, any more than heat rays which are invisible. Further, if red rays are a necessary constituent of white light, the colour blind should not be able to see white as we do, nor to distinguish white from bluish green — the complementary colour of red. And yet such distinctions can be made, although it may be argued that a colour-blind person does not see white in the same sense as white is white to a person having normal colour vision. A strong objection to the Young- Helmholtz theory is that in cases of colour blindness following injury to the eye, only the blue of the spectrum is seen, all the rest appearing as white. Here it is impossible to under- stand how a sensation of white can be experienced if the sensations of red and green are lost, for the theory is that white can only be experienced when the sensations of red, green, and violet are all three present. Stanley Hall likewise adopts an anatomical basis for his The Sense of Sight 165 .heory of colour perception. He holds that only the cones are sensitive to colours, and that these may be regarded as built up of a series of disks like a pile of coins, the lowest of which is the largest. Different disks respond to different colour tones, and give rise to different excitations of the nerve centres. While the disk formation of the cones is undoubted, this theory is open to the same objections, on subjective grounds, as that of Young and von Helmholtz. Other theories of colour perception proceed upon the assumption of chemical changes in the retina under the influence of light. That light does play an important part in physiological action is a well-known fact. Green plants, for instance, can only grow healthily when exposed to the light ; if kept in a dark chamber they quickly blanch, and use up only the reserve material stored up in themselves, because they have no longer the power of obtaining carbon from the carbonic acid of the air. And yet, though this is so, it is also known that direct rays of light have a retarding influence on the growth of certain parts of plants. If a plant is placed in a window, it bends outwards towards the light, because the side of the stem away from the light grows the faster ; similarly leaves of plants grown in the dark, like rhubarb, have long thin stalks which have derived their nourishment from the root, and have not been affected by light. So Hering holds with regard to the retina. According to Hering's theory certain fundamental sensa- tions are excited by light or by the absence of light. These are white, black, red, yellow, green, and blue, and they may be arranged in three pairs, the one colour in each pair being complementary to the other, thus — white to black, red to green, and yellow to blue. Hering further supposes that when rays of a certain wave-length fall on visual substances existing in the retina destructive changes occur, while rays having other wave-lengths cause constructive changes i66 Physiology of the Senses Thus, suppose a red-green visual substance exists of such a nature that when destructive and constructive changes occur no sensation is experienced, then when destructive changes are in excess by the action of light of a certain wave-length there is a sensation of red, and when constructive changes occur by the action of shorter waves the sensation is green. In like manner a yellow-blue visual substance by destruc- tive changes gives a sensation of yellow, and by construc- Yb \V gr V B G Y O FIG. 78. — Diagram to illustrate Bering's theory of colour vision. The vertical shading represents the red and green, and the horizontal shading the yellow and blue, antagonistic pairs of sensations. The thick line indicates the curve of the white sensation. All above the line X X indicates destructive changes in the retinal substances, and all below constructive changes. See text. (Report of the Committee of the Royal Society on Colour Vision.) tive changes a sensation of blue ; and a white-black visual substance by destructive changes gives white, and by constructive changes black. The member of each pair is thus antagonistic as well as complementary. The red-green and yellow-blue substances are tuned, as it were, to rays of different wave-length. Thus, in the red end of the spectrum, the rays cause great destruction of the red-green substance, while they have no effect on the yellow-blue substance. Hence the sensation is red. Again, the shorter waves The Sense of Sight 167 which correspond to the yellow of the spectrum cause great destruction of the yellow -blue substance, while their de- structive and constructive effects on the red-green substance neutralise each other. Hence the sensation is yellow. Still shorter waves, corresponding to green, cause construc- tion of the red-green substance, while their influence on the yellow-blue substance is neutral, and hence the sensation is green. Again, the shorter blue waves cause construction of the yellow-blue substance, while their action on the red- green substance is neutral, and hence the sensation is blue. At the blue end the short waves are supposed to cause destruction of the red-green substance, and thus give violet by adding red to blue. Orange is caused by excess of destructive changes, and greenish-blue by excess of con- structive changes in both substances. Finally, when all the rays of the spectrum fall on the retina, the constructive and destructive changes in the red-green and yellow-blue substances neutralise each other, but the destructive changes are great in the white-black substance, and we call the effect white. Colour blindness, in the form of red-green blindness, is, according to this theory, due to the absence of the red-green substance, the other two substances remaining. The phenomena of coloured after-images are thus accounted for : — Suppose the retina to be acted on by red light, destruc- tion of material takes place— the effect continuing, it may be for a time, after withdrawal of the red light — giving the positive after-image. Then comes the upbuilding of the material under the influence of nutrition, assisted by the action of light of shorter wave-lengths, and the negative after-image — green — is perceived. So with yellow and blue, and white and black. That such differences of chemical action are possible, or probable, we may well believe from a consideration of the variation in the actinic effect of different 1 68 Physiology of the Senses rays of the spectrum, and from the action of light upon the pigments of the retina. This theory is also in harmony with what has been observed in connection with many other processes in the body, such as secretion, innervation, and the like, in which tissues, having reached their highest point of vitality through nutrition, disintegrate during functional activity. Serious objections to the theory have, however, been raised. One is thus stated by Ladd : : — " A light composed of red and green may be made to seem to the eye the same as a light composed of yellow and blue. If, then, the eye is fatigued to red, instead of the red-green mixture appearing greenish, and so distinguishable from the yellow- blue mixture, they both appear the same to the fatigued eye." It has also been pointed out that the two sensations of each pair do not always coexist. One may be present and the other absent, Thus, when the intensity of the light of the spectrum has been much reduced, the green persists long after the red has disappeared ; and after the excessive use of tobacco, yellow may disappear, and blue is the only sensation left. One is also at a loss to understand how colour sensations, so different from one another as red and yellow, can be alike due to destructive changes of retinal substances, or how yellow and green, whose periods of vibration are so nearly alike, can give such antagonistic physiological effects. Such considerations demand the existence not of one but of three visual substances. On the whole, however, speculative as it is, Hering's theory accounts for a larger number of the phenomena of colour vision than that of Young and von Helmholtz. In a new edition of his great work, Handbuch der Physio- logischen Optik, now appearing in parts, von Helmholtz reviews the subject of colour vision, and materially modifies the theory as first announced by him about 1856, and since 1 J^add, Outlines of Physiological Psychology, p. 263. The Sense of Sight 169 then termed the Young- Helmholtz theory. He now states that luminosity or brightness plays a more important part in our perceptions of colour than has been supposed. He also, by analysing the colours of the spectrum with great care, lias been able, from these data, to determine three fundamental colour sensations, the first red (a), which is a highly saturated carmine-red, the second green (b) like the green of vegeta- tion, and the third blue (c) like ultramarine. Each spectral colour is made up of certain proportions of these funda- mental colours, or a combination of two of them added to a certain amount of white. Thus 100 parts of green are composed of 15 of a, 51 of £, and 34 of c ; or, to take other examples, spectral red will contain, per cent., 42 of a, i of b, and 57 of white ; yellow, 1 1 of a, 14 of £, and 75 of white ; and blue, 2 of a, 1 1 of ^, and 87 of white. The white gives the element of brightness. According to this view, it is not necessary to suppose that in the red-blind the red-perceiving elements are awanting, or that in the green-blind the green-perceiving elements are absent, but that these elements may be stimulated with intensities different from those affecting the normal eye. Suppose that in the eye of a colour-blind person the curves of inten- sity representing the red and green coincided, or, in other words, that the elements responsive to red and green in the abnormal eye were stimulated with intensities equal to that of red in a normal eye, the sensation would be yellow, as we find to be the case in so-called green blindness. Again, if in a similar way the red curve coincided as regards inten- sity with the green, the general effect would be that of a red-blind person, the red end of the spectrum would appear to be green, and no red would be visible. This theory does not profess to state what may occur in the retina in the way of chemical change, as is attempted in the theory of Hering. 170 Physiology of the Senses Captain Abney and Major-General Testing have also investigated the question of colour sensation by photo- metrical methods, and have been able to mark out the curves of luminosity both of normal and of colour-blind eyes. Their observations support the Young-Helmholtz theory, and indicate clearly that the peculiar sensations of colour experienced by colour-blind people are due either to the different intensities of the three primary colour sensa- tions, or to the absence of one or more of those sensa- tions.1 6. — BINOCULAR VISION Having considered the eye as an optical instrument, we have next to inquire how the two eyes act together, and what are the advantages of binocular over monocular vision. Movements of the Eye. — When we wish to change our field of view, we may do so either by moving the head as a whole, or the eyes alone. The eyes move very freely in their sockets, but, as we shall see, their movements have certain limitations. The orbits — the cavities of the skull in which the eyes are set — contain the muscles by which the eyes are moved, nerves, vessels, glands, connective tissue, and, lastly, a considerable quantity of fat, which forms an elastic cushion on which the eyeballs rest. The depth of setting of the eyes in the orbits varies in different people, and in the same person from time to time ; but. as a general rule, the eyes are so situated that one may, without moving the head, look outwards and slightly backwards to either side. We may readily prove this by standing erect with the back of the head against a wall. If some bright object on a level with the eyes, and touching the wall, be moved gradually outwards from the head, it will, at a certain point 1 Philosophical Transactions, 1886, 1888, 1892. The Sense of Sight 171 (about 8 inches to i foot), become visible. The head being kept fixed, a similar point may be determined for the other side of the head ; and a straight line drawn from these points through the outer angles of the orbits will be found to meet at an angle of about 90° ; or, in other words, if the head be considered as placed within a circle, only one quadrant of the circle is shut off from the visual field, namely, that in which the head lies. The movements of protrusion and retraction of the eye- balls are involuntary, and of little importance for vision, but rotatory movements of the eyeball require careful considera- tion. These take place round a .centre of rotation which, according to Bonders, lies 1-77 mm. behind the centre of the visual axis, or 16-05 mm- from the vertex of the cornea. We may conceive of three axes passing through this centre, an antcro-posterior, a FIG. 79. — Diagram to illustrate the fact transverse, and a vertical that we can see objects in a plane axis, and each of these axes behind a transverse vertical plane . . through the two eyes. may be regarded as lying in planes which, passing through the coats of the eyeball, divide the ball into two nearly equal parts, an upper and lower, an outer and inner, and an anterior and posterior. These axes and planes have a certain fixed position, \\\^. primary position, with reference to the orbit when the eye is at rest. If the eyeballs rotate on the antero- posterior or visual axes from the primary position, either vertically or horizontally, the eyes are said to have assumed a secondary position, and a tertiary position if they move in an oblique plane, so as to look inwards, and at the same time upwards or downwards. In the secondary position, there can 172 Physiology of the Senses be no rotation of the eye around the antero- posterior axis, but in the tertiary position there is always more or less rotation upon all three fundamental axes — on the antero-posterior, for example, it may be even more than 10°. Such circular rotation^ or rolling of the eyes, takes place when the head leans towards either shoulder. In this case the direction of rolling is such as tends to counteract the deviation of the head. The Ocular Muscles. — The movements of the eye are caused by the action of six muscles. Four of these, the direct muscles or rectt(¥\g. 80), pass forwards from the back part of the orbit to be inserted severally on the upper, lower, inner, and outer sides of the eye- ball, and their action is easily understood. When FIG. 80.— Diagram of muscles of right eye. the inner muscle contracts, i Elevator of the eyelid ; 2, superior fa Qn . oblique muscle ; 3, superior direct muscle ; 4, 4', external direct muscle cut in order tical axis and looks in- to show part of the optic nerve and ward and wh£n ^ ^^ 7, the internal direct muscle ; 5, inferior oblique muscle ; 6, inferior direct muscle, acts, Outwards. When the (Schwalbe.) upper contractSj the eyeball rotates upon its transverse or horizontal axis and the eye looks upwards ; when the lower contracts, the eye looks down. It must be borne in mind, however, that as the upper and lower recti pass somewhat obliquely outwards to their places of insertion in the eyeball, there is a slight inward direction given by them to the line of vision in addition to the deviation up or down. To correct the inward devia- tion, and, in general, to give circular rotation to the eye, two oblique muscles exist. The upper (superior oblique), The Sense of Sight 173 passing forwards along the inner wall of the orbit, passes through a small fibrous ring attached to the bone, and turns like a rope on a pulley backwards and outwards to be inserted into the upper surface of the eyeball. The other (inferior oblique), arising from the front part of the inner wall of the orbit, passes backwards and outwards under the eyeball, and is inserted into its outer part. The upper oblique muscle rotates the eye downwards and outwards, the lower upwards and outwards. The outer or inner direct muscle (external or internal rectus) alone suffices to rotate the eyes outwards or inwards in a horizontal plane. To cause upward or downward rotation vertically, the upper rectus and the lower oblique, or the lower rectus and upper oblique, come into play. For oblique movements, the two recti adjoining the quadrant, into which the fore part of the visual axis moves, together with one of the oblique muscles, act simultaneously. Further, since we habitually use both eyes in looking at an object, it will be readily understood how delicate and accurate the co-ordination of the muscular action must be. In looking upwards or downwards similar sets of muscles will of course come into play ; but in look- ing sideways the outer set of one orbit acts at the same time as the inner of the other, and, in converging the eyes upon a near object, the two inner sets will co-operate. The ocular muscles in all voluntary movements tend to render the view of the object we wish to look at distinct, by the formation of its image on the yellow spot, and they cannot act so as to lead to the formation of images on non-corre- sponding points of the retina (see p. 177). We cannot look upwards with one eye while the other eye is turned down- wards, nor can we look with the right eye to the right and the left eye to the left at the same moment. It has been pointed out by Le Conte that in drowsiness, intoxication, and death, when the eyes are in a purely passive state, the 174 Physiology of the Senses visual axes diverge slightly, and for this reason the intoxi- cated man sees double. Le Conte attributes this to the divergence of the axes of the orbits of the human skull, and holds it probable that " in a state of perfect relaxation or paralysis of the ocular muscles the optic axes coincide with the axes of the conical eye-sockets, and that it requires FIG. Si. — Vertical section through the left orbit and its contents in the orbital axis and with eyelids open, a, Frontal bone above orbit; b, upper jaw-bone below orbit ; c, thickened bone for eyebrow ; d upper, d' under eyelid with eyelashes ; e, e, meeting of conjunctivae of eyelid and eyeball ; f, muscle that elevates upper eyelid ; g, superior direct muscle ; g', inferior direct muscle ; /z, cross section through inferior oblique muscle ; i, optic nerve ; 2, cornea ; 3, anterior chamber ; 4, lens; 5, vitreous humour. (Allen Thomson.) some degree of muscular contraction to bring the optic axes to a state of parallelism, and still more to one of con- vergence, as in every voluntary act of sight." * The doubling of the image caused by external deviation of the fore part of the visual axes may be studied if we press upon the outer border of each eyeball with the fingers. All 1 Le Conte, Sight, p. 255. The Sense of Sight 175 objects in view are now seen double, and if the right eye be shut the left image disappears, and vice versd. How an Object is seen as One with Two Eyes.— When we look at an object in the far distance the antero- posterior axes of the eyes are parallel, and an image of the object will be formed upon the spot of distinct vision in each eye. Again, when the object viewed is near at hand, the visual axes converge, so that the image is still formed upon the yellow spot of both eyes, and the object is seen as single. This sensation of oneness arises from the habitual use of these areas of the retinas for the observation of one and the same point, and from the attention given to that point alone as distinguished from all others in the visual field. But if we displace one of the visual axes by pressing with the finger upon the corresponding eye we will seem to see all objects doubled, one image being stationary, the other moving as we vary the pressure. The reason for this is as follows : under ordinary circumstances the mind pro- jects the image formed in the eye outwards in the direc- tion of the visual axis, and this being now mechanically displaced the object seems to be in motion. But, further, since the whole field of normal vision seems single when seen with both eyes, it follows that the retinae, as a whole, act in combination, and give a single image of that which is focussed upon them. Now, suppose we hold two pencils upright in the middle plane of the body, but at different distances, we can voluntarily fix our attention upon one or other, and the one upon which we concentrate our regard will appear single, while the other will be indistinctly seen and will seem double. The image of that one to which we specially attend is single because the visual axes converge upon it, but the other is indistinct and double because its images on the two retinas are not in the line of regard, and not upon points which habitually act 176 Physiology of the Senses together. For each person there is always a certain visual Jield^ determined in shape by the outlines of the eyebrows, nose, and cheeks, and by the position of the eyes in regard to them, a field from each point of which rays entering the eyes always fall upon corresponding points in the two eyes. FIG. 82. — Binocular visual field. If a sheet of paper be held so as to touch the brow and prominence of the nose, the binocular visual field will be seen as in the space in I, bounded by the lines L and R. If the paper be held a few inches from the face the area visible to both eyes will have the shape seen in II. If, the head being fixed and both eyes open, the extent of the whole visual field be noted, and if the right and left eyes be alternately closed and opened, it will be found that the projection of the eyebrows and nose cuts off from each eye a certain part of the visual field which is visible to the The Sense of Sight 177 other eye, and that there is a central area common to both eyes, or a binocular visual field^ shaped as in Fig. 82. This area bears a fixed form and magnitude, and from it alone can rays of light enter both eyes. From each point in this field the rays of light entering the eyes must, for a given state of accommodation, fall upon the same points of the retinae. To each point, then, in the binocular visual field there is a corresponding point in each retina ; and, again, the right side of the right retina corresponds point for point with the right side of the left retina, and, similarly, the left side of the right retina corresponds with the left side of the left retina. Thus it follows that the upper halves correspond, and likewise the lower. The yellow spots form corresponding areas, and when the images of a small object formed upon these, and projected outwards by the mind upon the visual field, coincide in position the object is seen single. If, for example, the eyes are so directed that the images upon them of the point A (Fig. 83) are projected outwards so that the lines of projection meet at A, we will see A as one point, and any other point in its near vicinity, such as B, will likewise be seen single, because its images are formed upon corresponding points of the retinae. If we describe a circle whose circumference passes through the point of sight and the two optic centres, it may be mathe- matically shown that rays from all points of this circle fall upon corresponding points, and objects on it are seen N FIG. 83. — Diagram of one form of horopter. (Miiller.) 178 Physiology of the Senses single. Miiller called this circle the horopter \ and, for different positions of the eyes, the horopter may -assume complicated forms, but in any horopter all points 'are' seen single. We are now able to understand how a double image is seen when objects not in the horopter are seen double. Suppose in the case of looking at the pencils we represent the nearer one by p (Fig. 84), the farther by #'. Then, when the eyes are converged on^, the images of p' are not f FIG. 84. — Diagram to illustrate formation of homonomous double images. formed on corresponding points of the retinae, but are each to the inner side of the yellow spot at bb' , and two faint images of p' are seen, one on each side of, and at the same distance from, the eyes as /, viz. for the left eye at a, for the right eye at a. On shutting one or other eye, the image on the same side disappears, and it is said to be homo- nomous. But if the gaze be fixed upon p' (Fig.* 8 5) a double image of />, formed external to the yellow spot on both eyes, is mentally projected outwards to the distance of the plane a' a through p, and now on shutting one or other The Sense of Sight 179 eye the image -on the opposite side disappears, and it is hence said to be heteronomous. Now, as a rule, we are not conscious of the formation on the retina, nor does the mind project outwards this double linage. It is only by special attention to the action of both eyes that we become conscious of it ; and, at a first attempt, it is sometimes difficult to convince a person that a double image is, as in the above experiment, visible. The reason of this is, that attention is paid to the object directly looked at and not to the fainter double images ; and also FIG. 85.— Diagram to illustrate formation of heteronomous double images. because where we do try to see two objects at different distances at one and the same time, the minds of most people attend only to the image formed by the right eye and disregard that of the left. Thus, if you tell a person to point with the finger at a distant object, both eyes being open, and then ask him, while holding the hand steadily, to shut the right eye, he will seem to be pointing to the right of the object, and not directly at it ; but if he shuts his left eye he will seem to be pointing correctly. This applies more especially to right-handed persons, the reverse being 180 Physiology of the Senses the case with those who are left-handed. By careful observation, we can note the two images of the finger pointing, and may bring the more distant object between the images, and then, whether the right or left eye be shut, the finger will not seem to be pointing directly at the dis- tant point. Still another reason why we neglect double images is that these are often so large as to overlap one another, so as to be practically indistinguishable ; and the effect of the two combined in a psychical process by the mind is to lead to the perception of the third dimension' in space, or in other words, the perception of solidity. Perception of Solidity. — When we look at a solid body the images formed in the two eyes are not exactly the same, because the right and left eyes view it from different standpoints. This can be best appreciated by viewing some small object at no great distance from the eye, e.g. a book. If we alternately examine the book with the right and left eye, the other being meanwhile closed, and compare mentally the appearances presented to the two eyes, we observe that the right eye sees more of the right side of the book, the left more of the left. If we then note what area of background is hidden by the two images, we find that the part hidden from the right eye by the book is different from that for the left. Now, with both eyes open, let vision be accommodated for the background, but examine the effect produced by the interposition of the book. We are then conscious of a solid opaque body obscuring part of the background completely, while to either side of this is a spectral transparent image of the sides of the book through which the wall seems to be seen. On shutting the left eye the solid body seems to move to the left, rendering the left spectral part opaque, because the part of the wall formerly seen by the left eye is no longer visible, and similarly for the right. It will further The Sense of Sight 181 be noted, as we converge the eyes on the book, that the spectral parts disappear, and we see the one solid body only. Lastly, if we look at the book fixedly for some time, one eye being shut, and then if we look with both eyes, it is at once seen that the book stands out in much bolder relief, the various sides and borders taking their natural inclination in reference to space. A suitable object for the study of this phenomenon is a truncated pyramid upon which we look vertically downward. With both eyes open the appearance presented is that seen in B (Fig. 86). Keeping the head in the same position, but looking with the left eye B Z FIG. 86. — Appearance of a truncated pyramid seen from above with B, both eyes, L, left eye, or R, right eye. only, we will see the pyramid as in L, or with the right eye only, as in R. The Stereoscope. — The combination of L and R, so as to present the appearance of solidity to the eye, may be made by the stereoscope, an instrument invented by Wheat- stone, who first noticed that the perception of solidity was due to the dissimilarity of the images presented to the retinae. In its simplest form the reflecting stereoscope consists of two mirrors placed at right angles to each other, as in P"ig. 87. The eyes, looking into these obliquely, see reflections of the dissimilar figures R and L representing the appearances as1 seen by each eye individually ; and 1 82 Physiology of the Senses the images, mentally projected backwards in the line of vision, are combined at the point of intersection of the optic axes, and we seem to see the single solid object as we would if we were looking at it with both eyes. Brewster's refracting stereoscope is the one in common use. In this instrument the optical effect is obtained by means of two lenses so arranged that rays of light passing from the stereoscopic pictures impinge on the retina, and are projected backwards so as to converge and meet at points FIG. 87. — Wheatstone's stereoscope. behind the plane of the pictures, as in Fig. 88. Each eye thus sees its own picture, but corresponding points are brought to a focus, and in the union of all we have one picture in relief. The apparently differing distances from the eye of different parts of the combined picture are due to the differing distances between corresponding points of the constituent pictures. Those pairs of points which are nearest together stand out in highest relief, or in other words, require the greatest convergence of the optic axes, while those which are most distant from one another seem The Sense of Sight 183 most remote in the combined picture. In Fig. 86, p. 181, XYZ the points , ' ' are respectively at equal distances from one another, and consequently seem to be in the same plane in B. Similarly f' yj ZfJ are at equal distances from one another, and seem to be all in one plane, but the dis- ..X.. * y FIG. 88. — Diagram illustrating the principle of Brewster's stereoscope. The points x, x forming images x1 , x' are projected outwards and coincide at X ; the points y, y, being nearer to one another than x, x, appear to coincide at a point Y in a plane nearer to the eyes than X. (After Landois and Stirling.) tance between any pair of these being less than the distance •y- . -y 7 between any pair of the set ', ' v,' ', the plane xys A , Y , L , seems nearer than the plane XYZ. Hence the trun- cated apex of the pyramid seems nearer the eye than the base. But if we transpose R and L so that R is opposite the left eye and L opposite the right, then the points ,' , , will respectively be farther from each other than •'•"> y i z XYZ ,' ' ' and we seem to be looking into a hollow pyramid, A, Y , Z, 184 Physiology of the Senses whose apex is directed away from us. In Fig. 88 the points #•, .r, being farther apart than y, y, are combined at X in a plane behind that through Y, the point of combination of y,y> It is indeed unnecessary to have a stereoscope to get the combined effect. If we merely fix the eyes upon the diagram, but accommodate the vision for distance, we will see the two diagrams apparently moving towards each other and overlapping until they seem to coincide, when suddenly the effect of a solid body between two faintly visible flat diagrams is perceived. Ordinary stereoscopic pictures are obtained by taking photographs of the same scene from slightly different standpoints, corresponding to the distance between the right and left eyes. These are fixed- to a card in their proper relationship to the right and left eye ; and if reversed, they give an inverted picture, all solid bodies seeming to be hollow. Even with the pictures properly placed it is possible, by a simple arrange- ment of lenses, as in the instrument called the pseudoscope, to displace the picture so that our judgment of the size of objects is disturbed by the apparent alteration in their distance from us. The Telestereoscope. — The stereoscopic effect depend- ing upon the distance between the eyes, it will naturally be greater, the greater the distance. We cannot, indeed, increase the distance between the eyes, although a small solid body stands out in higher relief when near the eyes than when far away, because the visual axes are more convergent. But von Helmholtz has invented an ingenious instrument by which the eyes are virtually separated and a more powerful stereoscopic effect obtained. It is known as the psetidoscope or telestereoscope, and the principle of its construction is as follows. Two mirrors are placed parallel and a little to the side of the mirrors used in Wheatstone's The Sense of Sight 185 stereoscope (Fig. 89). The rays from the object to the outer mirrors are reflected to the inner mirrors, and thence to the eyes. It thus happens that rays falling on mirrors much more distant from each other than the eyes, enter the eyes as if coming directly to them from the object. We are thus able to see, as it were, more of the sides of the body than we could under ordinary circumstances ; distant objects seem to be brought nearer, judging by their greater relief, and all parts of the field likewise stand out in a more marked manner than usual. In viewing the different parts of a solid body, or the apparently nearer and more remote parts of a stereoscopic picture, there is a constant movement of convergence or divergence of the eyes, and hence it was maintained that a prime factor in the percep- tion of solidity is the sense of muscular effort required in moving the eyes from point to point. This theory, how- ever, is negatived by the fact that we have quite a correct perception of the spatial relations of objects when seen by the instantaneous flash of lightning, a flash which takes place so rapidly that there is no time for all the complicated processes involved in muscular action. Similarly, the stereoscopic effect is seen when the picture is seen by the light of the electric spark ; that is to say, in a time not exceeding the 24^00 part of a second. But though the time of stimulation of the retina is momentary, there is an appreciable time lost in the physical change of the condition of the retina, in the passage of the nerve current, in the arousing of sensation and the fusion of the stimuli. Wheatstone held that, 1 86 Physiology of the Senses in the fusion of two images not mathematically similar, the mind superadds the perception of solidity. If the points in the two pictures are so far apart that the con- verging apparatus is unable to bring them to a focus, we only see two flat pictures. If the two pictures are exactly similar, and their points may be exactly fused, the result is a flat picture. The mental fusion is the cause of the new sensation. The fusion in ordinary circumstances is to all intents and purposes complete. The external world presents itself to us with each object clearly single and defined. It is only when we pay close attention and carefully analyse our visual sensations that we can detect the fact of incomplete fusion. We have, for example, the sensation of luminosity. When carefully examined this is found to be due to the irregular reflection of rays of light from the uneven surface of a body ; calm water is non -luminous, rippling water sparkles with light, but the amount of light going from the broken surface to one eye differs from that going to the other, and the effort at fusion of the darker and the lighter gives rise to the sensation of luminosity. The combined stereoscopic picture is luminous from the superposition of darker and lighter spots in the one picture, or the reverse in the other. And yet the fusion is incomplete when we look into the matter closely. By an effort of will we can allow the dark or the light to preponderate. Suppose we have two stereoscopic pictures, as in Fig. 90, one of which is light on a dark ground, the other dark on a light ground, we can, by a voluntary effort, superpose the one over the other and give rise to the impression of a luminous solid body ; but we can also easily alter the depth of the grayish luminosity by paying attention to the dark or the light picture at will. We have here, indeed, an analogy to the detection The Sense of Sight 187 by the ear of the elements of a compound tone. The practised ear is able to separate and attend to any one elementary tone, or, on a larger scale, to any individual instrument in an orchestra ; and the mind may dwell only on the harmonious fusion, experiencing a pleasure from the combination, or it may give itself up at will to the effect of one or of all. The process is easier with the ear than with the eye. The optical fusion is more complete, more diffi- cult to analyse. But it may be made easier if we endeavour to fuse two surfaces of different colours in the stereoscope. Here there is not complete mixing of the colours, but the colour sensation is now that of one, now that of the other FIG. 90.— Diagram to illustrate causation of sensation of luminosity. colour, the varying effect being probably due to changes in the activity of the two retinae. Estimation of Distance. — The foregoing considerations on the perception of solidity will assist us in answering the more general question as to the estimation of space or distance. We have seen that the muscular effort at con- vergence is greater for near than for remote objects, and the greater the effort experienced the nearer do we judge the object to be. But accompanying the effort at converg- ence there is usually a muscular action of accommodation. The pupil contracts to shut off divergent rays of light which would cause blurring of the image, and the ciliary muscle contracts in order to lengthen the focal distance of the eye 1 88 Physiology of the Senses for the nearer object. Each of these muscular efforts must add its quantum to the general sum of muscular sensation. Objects at the point of sight are seen in clear detail, while those which are nearer or farther off are seen indistinctly, and we unconsciously judge of differing distances by varying efforts of accommodation. The dimness of bodies within the near point of vision is due to the impossibility of focuss- ing the object. Far-distant objects are dimly seen because of the aerial perspective. The atmosphere not being per- fectly transparent and colourless, small details are blotted out, and variety of colour lost in a bluish haze. The dis- tant parts of a landscape are conceived to be nearer and smaller when seen in wet weather than in dry, for dust- laden air gives a more marked aerial perspective than that which has been washed by rain ; and again, in misty weather the half-hidden forms of men may seem far away and of supernatural size. Again, varying convergence assists our estimation of distance, not only through the muscular effort involved, but also by variation of the angle of convergence of the visual axes upon the object. For objects of similar size it is evident that the angle of convergence must be greater for near than for remote objects. We learn through the other senses, as well as through sight, to know the com- parative sizes of objects, and by noting and comparing the apparent size of objects we arrive at a judgment as to their distance, the seemingly smaller, of course, being considered the more distant. Persons who have lost the use of one eye, and therefore the valuable aid of convergence, cannot judge accurately of the distance of near objects. If asked to touch an object quickly they are apt to fall short, as ex- perience tells them they may misjudge and strike it roughly if they attempt to reach the full apparent distance. Estimation of distance is likewise assisted by observation The Sense of Sight 189 of the distance of the background over which a body near to the eye seems to move when the relative positions of the eye and the body are changed. In Fig. 91, I. the eye E moves while the body B is stationary, in II. the body moves from B to B' while the eye is stationary. The apparent distance moved by B upon XY is only ab, while upon X'Y' it is the much larger distance ab '. The distance over which the body seems to pass gives an indication of the relative distances of the planes XY, X'Y' from the observer. We are also able to give a more accurate estimate ot X'— £ -£ Y' X'- II Fio. 91. — Estimation of distance from change in relative position of the eye and of an object observed. the distance between two points when several objects intervene. We take a series of mental leaps, as it were, from point to point, the effort of which is greater than that of passing over the whole distance at one effort. The dis- tance between A and B (Fig. 92) seems greater than that between B and C on account of the intervening dots, but it » « « « « * • o is the same. Children often FlG 92 amuse themselves with the following experiment. A boy, after looking at a landscape in an erect posture, will turn, stoop down, and view it between his legs, and all objects will seem farther off, as, from the unaccustomed posture and the proximity of the 190 Physiology of the Senses head to the ground, objects in the foreground, formerly dis- regarded, are now more dwelt upon. Similarly, the sky seems nearer us at the zenith than at the horizon, and a landsman has great difficulty of judging distances at sea. The eye projects the image of the object viewed outwards, but if it be at any great distance, the lines of projection from the two eyes are practically parallel, and judgments as to size guide the judgment as to distance. It is interest- ing to note, in this regard, that persons who have been born blind and have by an operation gained the power of vision, seem at first to see all objects close to the -eye or almost touching it — they "see men as trees walking" — and it is only after a process of education in which the sense of FIG. 93. — a and b are of the same length, but b subtends a greater visual angle, being nearer to the eye. touch has much to do that they are able to form a proper estimate of externality or distance through vision. Estimation of Size. — Closely connected with our esti- mate of distance is that of size. This primarily depends on the size of the retinal image, or in other words, of the visual angle subtended by the object. In Fig. 72, p. 146, X is the visual angle subtended by the lines c, d, and e of a simple globular shape, or, in highly developed animals, it may assume a very com- plicated form ; so much is this the case in man, that it is known as the membranous labyrinth. The structure of the labyrinth is, as we shall see, of a most delicate and elaborate nature, and though in the embryonic condition it is near the surface of the head, in the adult it is at least i^ inch from the surface, and enclosed in bone so hard that it is called the petrous or stony bone. The osseous covering coincides to a great extent with the membranous bag inside, but a small amount of fluid separates the sac from its walls, and protects it from rude shocks transmitted through the bone. The auditory cells are situated in certain parts of this sac, and FIG. 99. — Auditory vesicle of Phiahdium. d\, «o> the auditory nerve Epithelium covering the sac ; h, auditory cells, passes tO them through with hh auditory hairs ; «/, nervous cushion for the auditory cells, connected with nr\, the lower Channels in the bone. nerve r;ng. (Hertwig and Lankester.) There are also two openings by which changes of pressure may be transmitted from without to the fluid surrounding, and that contained by, the membranous labyrinth. But these openings cannot be seen from the outside. They communicate with a chamber known as the middle part of the ear, or simply the middle ear, or tympanum, or drum, a chamber containing air and opening by a tube passing forwards and inwards into the throat — the Eustachian tube. The middle ear is separated from the passage leading to the auricle, or visible ear, by a mem- brane, known as the membrana tympani (or drum-head}, which vibrates in response to sounds, and whose move- 200 Physiology of the Senses ments are communicated to a chain of bones, and by this chain to the inner ear. The membrana tympani closes the passage leading inwards from the outer ear or auricle. There are thus an outer and middle ear for the collection and transmission of sounds, and an inner ear for their reception as stimuli of sensation. By this arrangement the ear becomes more sensitive, for the middle ear acts as a drum giving resonance and strength to delicate sounds (Fig. 101). In order to obtain a complete understanding of the manner in which sound affects the ear, we must consider carefully the structure of the ear, and how it is fitted to respond to sonorous vibrations. i . External Ear. — The shape of the external ear varies to a remarkable degree, and in some of the lower forms of vertebrates it may be entirely absent. In the frog, for example, there is no external ear, the tympanic membrane being seen as a disc on a level, and continuous with the skin of the head. In birds, again, the auricle is absent, but there is an external auditory canal or meatus leading down to a membrana tympani. The middle and internal ears are more highly developed in birds than in reptiles, but still fall far short of the human ear in complexity. In mammals, the auricle is of very varied size and shape, and it may be either stiff and erect from the presence of an elastic cartilage, as in the ear of the horse or man, or it may be soft and yielding, as in the elephant. The surface is usually convoluted and funnel or trumpet shaped, so as to gather the waves of sound to the best advantage, and many animals have the power of moving the opening of the auricle, by means of voluntary muscles, in the direction from which the sound comes. Thus the horse pricks up its ears when it hears a sound, and no doubt its appreciation of the direc- tion of sounds is thereby assisted. In the human ear there are similar voluntary muscles, but man has, for the most Sound and Hearing 201 part, ceased to have the power of moving the auricle in response to sounds from varying sources apart from move- ments of the head as a whole. No doubt, by attention and practice, a man may acquire the power of moving the auricle slightly, and the great German physiologist, M tiller, was proud of being able to do so. But, at best, these movements are small as compared with those of the lower animals. Special names have been given to the various depressions and protuberances of the auricle (for which see description of Fig. 100). If we pass the finger round the border of the ear we will feel near the upper part a small nodule, which is interesting, according to the com- parative anatomists, as being homo- logous with the tip of the pointed ear of many animals. The general effect of the con- volutions of the surface of the auricle is to collect and transmit to the ex- FIG. «oa— Outer sorfeoe of the ternal auditory canal, and that to the risht auride- i, Helix ; 2> fossa of helix ; 3, antihelix ; best advantage, sound waves falling 4i fossa of the antihelix ; upon the surface of the ear. For 5, antitragus ; 6, tragus ; 7, ,. .. . .. „. concha; 8, lobule. (Arnold.) just as waves of light falling upon a transparent body are partly reflected and partly trans- mitted, so sound waves striking the auricle are partly concerned in giving rise to corresponding vibrations in the substance of the auricle, and partly reflected, and the more the waves are sent to the inner ear the more intense will be the sound. The phenomenon familiar to every one, of the echo, is an example of this reflection of sound on a large scale in nature. We hear first the sounds trans- mitted directly to the ear, then those reflected from more 202 Physiology of the Senses or less distant bodies. In the whispering gallery of St. Paul's Cathedral in London, or in the ducal mausoleum at Hamilton, faint sounds can be heard at a considerable dis- tance from the point at which they originate, as they are reflected in such a way as to be focussed at a special point. So the shape of the auricle, by focussing sounds, helps the ear to hear sounds of low intensity. It would appear also that the form and size of the depressions of the concha strengthen tones of very high pitch, such as occur in hissing sounds, like the noise of waves breaking on a shingle beach, or that of a waterfall. Thus a very slight change in these depres- sions will affect the musical quality of tones. If the irregu- larities of the surface are filled with wax, sounds are not heard so loudly, and, conversely, we increase our receptivity by putting the hand to the ear, and turning the head side- ways to the sound. If the auricle is entirely removed, hearing is, however, but little diminished. The collecting power of the auricles assists in the determination of the direction from which a sound comes ; the sound being more loudly heard in one ear than the other, we conclude that it comes towards that side of the head on which the louder sound is heard. 2. Meatus or Passage. — From the pinna or auricle, the external auditory meattis, or passage to the middle ear, passes inwards and slightly forwards, being inclined at first upwards and then bending downwards. The passage is almost circular in cross section, but the outer end is flattened a little from before backwards, while the inner part is broadest in the horizontal plane. The meatus is closed internally by the tympanic membrane, or drum-head (see Fig. 101, 17), which lies obliquely to the direction of the lumen of the tube, the lower margin being farther in than the upper, and the floor of the passage is thus longer than the roof. Sound and Hearing 203 The wall of the outer part of the meatus consists of cartilage which is continuous with that of the auricle, but round the deeper part of the tube the cartilage is absent, and the lining of skin which passes inwards from without is in close contact with the bone through which the tube FIG. 101. — Diagram of the ear ; natural size, i, Auditory nerve ; 2, internal audi- tory meatus closed by the cribriform plate of bone through the perforations of which the branches of the auditory nerve pass to the ear ; 3-8, membranous labyrinth composed of 3, utricle, 4, semicircular canals, 5, saccule, 6, duct of the cochlea (the coils not entirely shown), 7, endolymphatic duct with, 8, its saccule lying inside of the cranial cavity ; 9, lymphatic space surrounding the membranous labyrinth ; 10, osseous labyrinth of compact bone lying in the more spongy substance of the petrous bone, n, n ; 12, the oval window, filled by the foot-plate of the stirrup-bone ; 13, the round window, across which is stretched the internal tympanic membrane ; 14, auricle ; 15, 16, external auditory meatus ; 15, its cartilaginous, and, 16, its bony part ; 17, tympanic membrane; 18-20, auditory ossicles; 18, hammer; 19, anvil; 20, stirrup ; 21, middle ear ; 22, osseous, and, 23, cartilaginous portion of the Eustachian tube ; 24, cartilages of external auditory meatus. (Schwalbe.) passes. Towards the inner part of the meatus the skin is very thin, and this is especially the case where it is con- tinued as an epidermic covering over the fibrous tympanic membrane. At the outer part the skin is thicker, and from it spring fine hairs slanting outwards. It is well lubricated 2O4 Physiology of the Senses by numerous small glands, of the nature of sweat glands much modified, which secrete a waxy substance known as cerumen. This material has a brownish colour and a bitter taste. The form of the canal is such as to facilitate the passage outwards of the wax, but sometimes it may accumu- late in such quantity as to diminish the power of hearing to a considerable e'xtent. If this should happen, a sharp hard instrument should not be employed for its removal, as much injury might thereby be inflicted upon the tympanic mem- brane. It is better to soften the wax with an alkaline or oily fluid, and then to syringe the meatus gently to remove the debris. The outward -pointing hairs and the bitter adhesive wax form together a valuable guard against the entrance of foreign bodies, animate or inanimate, into the cavity of the meatus, a provision similar to what we find in many flowers to prevent the store of honey from being plundered by marauding insects. 3. The Middle Ear. — The middle ear, drum, or tym- panum is, in the adult, about an inch and a quarter from the free surface, and is thus embedded deeply in the sub- stance of the temporal bone. Across this space passes the chain of bones from the drum-head to the internal ear, by means of which the movements of the membrane are trans- mitted to the labyrinth and variations of pressure effected. It receives air at atmospheric pressure through the Eus- tachian tube. The cavity is irregularly wedge-shaped, being wider at the top than at the bottom, and is .larger from before backwards than from side to side. It is separated from the cranial cavity above by a thin layer of hard bone, and communicates behind with a set of spaces, which also contain air, lying in the part of the bone which can be felt as a prominence behind the external ear, and known as the mastoid process. The outer boundary of the middle ear is largely composed of the tympanic membrane, although it is Sound and Hearing 205 a to be noted that the cavity extends upwards into the bone above the membrane, while in front of the membrane is a fissure in the bone, known as the fissure of Glaser, from its discoverer, through which pass a nerve (the chorda tym- pani) and a muscle (the laxator tympani), and in which, as in a socket, is fixed one of the processes by which the chain of bones is suspended. The membrane itself is firmly fixed in a groove, which can be readily seen in a macerated bone with the naked eye, and, though very thin and semi-trans- parent, it consists of firm fibrous tissue lined on one side by skin, on the other by mucous membrane. Fig. 102 represents the appear- ance of the tympanic mem- brane of the left ear as seen from without, and Fig. 101 shows how it is inclined obliquely to the axis of the meatus, both transversely, and from above downwards. The fibres of the mem- brane consist of ordinary connective, and a very small amount of elastic, tissue, and are disposed in a two- fold manner, some of them radiating from a point, the umbo, slightly below the centre of the membrane to the circumference, while others are arranged concentrically around the same point. The outer surface of the membrane is covered by a very thin layer of skin, while its inner tym- panic surface is lined by ciliated epithelium, The first of FIG. 102. — Left tympanic membrane show- ing the arrangement of its fibres, a. anterior, b posterior border ; i, flaccid part of the membrane ; 2, short process of the malleus ; 3, umbo of the mem- brane ; between 2 and 3, the handle of the malleus ; 4, anterior and, 5, posterior end of the tympanic groove, between which are seen circular fibres attached to the short process, 2. (Schwalbe.) 206 Physiology of the Senses the chain of bones is firmly attached to the fibrous part of the membrane in such a way that the central part of the membrane is drawn inwards towards the tympanum, form- ing the umbo (or boss of a shield), and thus the disc is not flat, but slightly conical, and, owing to the circular fibres, the surface towards the meatus is convex. This cur- vature of the membrane, though slight, is of considerable import- ance in connection with the re- sponse of the membrane to sonorous vibrations. The sound waves fall FIG. 103. — Horizontal section On the convex surfaces of the ilSt&S&S "dating fibres- These k-P where which is seen the handle of the the grOOVC of attachment is de- malleuscK across ; ,, anterior fi . d he membrane js loose wall of the tympanum ; f, in the tympanum above the thicker, and more freely supplied stapes whose base is inserted wj h neryes &nd blood.vessels. into the fenestra ovahs ; q, the stapedins muscle ; h, por- The inner wall of the tympanum, tion of facial nerve ; «, tensor opposite the membrane, is irregular tynipam muscle ; k, vestibular division, and, /, cochlear divi- in shape, and perforated by two sion of the auditory nerve apertures. The Upper of these, the lying in the internal auditory meatus ; **, cochlea ; «, nerve fenestra OVdllS, Or OVOl Window, IS going to ampuihe of semi- of an ovojd or kidney shape, and circular canals ; o, section of utricle ; /, section of sac- has the inner end of the ossicles cule; r, section of semicircular Qf the ear fastened into it by canals. (Riidinger.) means of a ligamentous tissue. The fenestra ovalis opens from the middle ear into the vestibule of the labyrinth. Lower down there is a smaller and more rounded aperture, the fenestra rotunda^ or round window, leading into the front part of the labyrinth, Sound and Hearing 207 known as the cochlea, but closed during life by a thin mem- brane like the membrana tympani — that is to say, com- posed of fibrous structure, with an epithelial lining upon either side, and having a slight concavity towards the tympanum (Fig. 101, 13). Between and in front of the above-mentioned apertures is a rounded elevation called the promontory, which corre- sponds to the first turn of the cochlea (p. 228). Behind the oval window is a very small process of bone perforated to allow the passage of a minute tendon, which gives attach- ment to the stapes *• (p. 2 1 1 ) of a small muscle, the stape- dius, the belly of which lies in a space behind the tym- panum (Fig. 103,^). The . passage leading away from the front of the tym- panum is divided into two parts by a little ledge of bone, known as the proccssus cochleariformis, the upper part con- taining the fleshy part of a muscle, the tensor tympani, whose tendon crosses the tympanum to be inserted into the malleus, the lower going forwards as the Eustachian tube (Fig. 103, z). The Eustachian Tube. — The mucous membrane of the Eustachian tube is continuous behind with that of the. FIG. 104. — Incus and malleus of the right side seen in their natural position in the tympanum, i, Tympanic membrane ; 2, Eustachian tube ; 3, tensor tympani muscle seen attached to the malleus ; 4, anterior ligament of the malleus attached to the processus gracilis ; 5, superior ligament of the malleus ; 6, chorda tympani nerve ; a, 6, c, sinuses or spaces connected with the tympanum in which the ossicles move freely. (Schwalbe.) 208 Physiology of the Senses tympanum, in front with the pharynx or upper part of the throat. When, under certain conditions, this mucous mem- brane becomes swollen, the lumen of the tube may be blocked, and air does not pass readily to and fro between the throat and the middle ear. Then the pressures upon opposite sides of the membrane becoming different, the membrane is too much stretched, does not respond so well as usual to sonorous vibrations, and one becomes slightly deaf. It is commonly held that the Eustachian tube is open only during swallowing, and the positive and negative experiments of Valsalva are brought forward in proof of this. The positive experiment is performed as follows : Close the mouth and nostrils, and then, while making the movements of a forced expiration, swallow. The air in the pharynx is at more than atmospheric pressure, but does not force its way into the tympanum until the tube is opened during swallowing. Then the condensed air pene- trates into the middle ear, raises the pressure there, and the drum-head is forced slightly outwards and made more taut. The tightening of the membrane gives rise to a peculiar sensation referred to the region of the ears, and similar to what is sometimes felt after yawning. We may directly observe this movement by inspection of the membrane during the act. The principle of the negative experiment is much the same. Instead, however, of making a forced expiration, we close the mouth and nostrils, raise the chest as in forced inspiration, and swallow. The air in the throat being at less than atmospheric press- ure, when the Eustachian tube is opened the pressure in the middle ear is reduced, and the tympanic membrane moves inwards by the atmospheric pressure in the meatus. We have also met with a gentleman who had the voluntary control of the tube, so that he could open or close it at pleasure. The advantage of having the tube closed at all Sound and Hearing 209 times, except when we swallow, lies in this, that were it always open there would be too much reverberation caused in our ears by the sound of our own voice. This, however, cannot affect the ears during swallowing, because then the lower part of the pharynx is cut off from the openings to the nose and ears by the meeting of opposite muscles, and the lifting of the uvula and soft palate. From all this it follows that one, and probably the most important, function of the Eustachian tube is to equalise atmospheric pressure on the two sides of the drum-head. The Chain of Bones. — Across the cavity of the tympanum stretches the chain of little bones or ossicles (Fig. 104), to which frequent reference has already been made. This corresponds to the single bone in the frog's ear, which stretches from the tympanic membrane T FIG. 105.— The malleus to the entrance to the inner ear, but, as or hammer bone seen we shall see, the chain confers consider- I""0"} in front- x» The head ; 2, the processus able mechanical advantage. It consists gracilis foreshortened ; from without inwards of the malleus or hammer bone, the incus or anvil bone, and the stapes or stirrup bone. The body or head of the malleus (Fig. 105) is situated above the level of the tympanic membrane, and it gives off downwards a comparatively strong process, the handle of the hammer, which is firmly affixed to the fibrous layer of the membrane. And just as a flattened beam will bear a greater downward pressure when placed edgewise than when laid flat, so the handle of the malleus, being flattened, is placed edgewise towards the tympanic membrane, thus combining lightness with power. Another process, the pro- 3, the short process . 4, the manubrium in- serted into the tym- panic membrane. The surface of the joint with the incus is not seen, as it faces back- wards. (Schwalbe.) 210 Physiology of the Senses cessus gracilis more slender and elongated than the handle, passes forwards from the junction of the head with the handle, and is firmly fixed by ligaments to the little fissure in the bone in front of the tympanic membrane. This pro- cess is of interest as constituting one end of the axis upon which the chain of bones rotates. The head of the malleus is rounded, and attached to the roof of the tympanum by a 4 small ligament. It bears upon its pos- 4 terior aspect a smooth surface for arti- culation with the incus. The head- is connected with the handle by a con- stricted neck, immediately below which we find, on the inner side of the handle, the point of attachment of the tensor tympani muscle, and on the outer part FIG. io6.-~Right incus a small bony prominence which, im- or anvil bone, x 4. i, pinging upon the tympanic membrane, Body ; 2, joint surface . j r i for malleus; 3, projec- causes a projection outwards of the tion that locks with membrane at that point. The laxator malleus to prevent over . . . , r movement ; 4, short tympani muscle passes backwards from process for posterior the fissure of Glaser, to be attached to attachment of the , •, c ,-, ,-, • •, .-, bone; 5, elliptic area the neck °f the malleus, JUSt above the on median side of short origin of the processus gracilis. Th<= -««• <"• anvil-shaped bone (Fig. cular knob ; 7, entrance 106), lies behind the malleus, and is Jointed to k by a saddle-shaped surface. A short process, pointing backwards, and fixed to the posterior wall of the tympanum by ligaments, forms the posterior end of the axis of rotation of the chain of bones. A longer process, corresponding to the conical pro- jection of an anvil, points almost vertically downwards, but,- at its lower extremity, bends inwards and ends as a little flattened knob, the lentictdar process, which in early life is a separate bone, the lenticular bone. A small eminence, Sound and Hearing 211 immediately below the surface of articulation with the malleus, should be noted, as it fits into a corresponding depression in the malleus and prevents undue rotation. The stapes, or stirrup-shaped bone (Fig. 107), is fixed in a horizontal plane, and at right angles to the descending process of the incus. The head of the stirrup is jointed to the lenticular process of the incus. Inwards from the head is a slight constriction; the neck, and from this arise the two arms of the stirrup. These are fixed at their inner end into an oval -shaped plate of bone, the base of the stirrup, which again fits into the oval window. The stirrup could move out- wards and inwards freely but for the firm short fibres which unite its base to the 2- margins of the aperture. The space be- tween the arms is filled during life by a FIG. 107.— Stapes or thin membrane, the arms being grooved stirrup bone seen to receive it. By this arrangement, light- from above> x 4- '. Base ; 2 anterior, ness and strength are secured in the same 3 posterior limb; way as we make wheels with spokes instead 4i head ; 5, neck ; 6, groove into which Of SOlld dlSCS. The tendon Of the Stape- membrane is fixed dius muscle is attached to the back part of which fills the open- ing. (Schwalbe.) the neck of the stapes. Movements of the Bones. — The malleus and incus rotate almost as one bone on a horizontal axis, passing fore and aft between the attachments of the slender process of the malleus in front, and the short process of the incus behind. The plane of rotation is consequently at right angles to that of the tympanic membrane, or across the cavity of the tympanum. When, then, the handle of the malleus is pushed inwards towards the mesial plane of the head, the head of the malleus moves outwards, carrying with it the body of the incus, any excess of movement being prevented by the suspensory ligament of the malleus. The 212 Physiology of the Senses body of the incus rotating outwards, its descending process moves inwards synchronously with, and parallel to, the handle of the malleus, and the tip of the process is thus moved inwards and slightly upwards, and pushes the base of the stapes into the fenestra ovalis. There is also a slight rotation of the stapes in a vertical plane, and the upper border of the base of the stapes has a somewhat greater movement than the under side. We see, then, that when, by compression of the air in the external meatus, the tym- panic membrane is forced inwards, the base of the stapes will also be forced inwards, and the pressure on the internal ear will be increased. Again, when the air of the external meatus is rarefied, and the pressure on the inner side of the membrane becomes greater than on the outside, the membrane is forced out- wards, carrying with it the handle of the malleus. Then the head of the malleus above the axis rotates inwards, carrying with it the body of the incus, and the long process of the incus, moving away from the mesial plane, carries the stapes with it, and pressure on the internal ear is diminished. The distance through which the base of the stapes can move is very small, and hence it might happen that a very loud sound, causing the tympanic membrane to vibrate through a comparatively large distance, might tear the stapes from its attachments. This, however, is guarded against in several ways. In the first place, a somewhat dense ligament passes from the upper part of the external wall of the tympanum to the head of the malleus, and this receives the impact of the head of the malleus as upon an elastic cushion, and may, when the head of the malleus tends to move too far inwards, restrain it from moving too freely. Secondly, the process below the upper joint of the incus fits into a depression in the malleus, and when the handle of the malleus tends to move too far inwards, this Sound and Hearing 213 projection locks into the opposing socket like the tooth of a cog-wheel, and prevents too great movement inwards. On the other hand, if the handle of the malleus rotates outwards excessively the tooth is withdrawn, and the saddle-shaped joint coming into play, the lower part of the joint tends to gape, and the incus does not move so far outwards as the malleus. Further, the chain of bones acts like a bent lever, the arm of the incus being only two-thirds of the length of the malleus. When the lower end of the handle of the malleus, fixed in the umbo of the tympanic membrane, moves through a given distance, the stapes fixed to the lower end of the process of the incus will only move through two-thirds of this dis- tance. But while the excursion distance is diminished, we know from the principle of the lever that the force with which it FlG- 108.— Diagram . illustrating the moves must be increased by one-half. There leverage action of is thus diminished amplitude of movement, the malleus and _. . ,. . incus; w, handle but increase of power. This is a distinct Of malleus ;/, long advantage, considering the small power that process of the r . , . incus. sound waves have of moving the tympanic membrane, and the firmness with which the base of the stapes is fixed. This increase of power is augmented by the fact that the tympanic membrane has roughly an area twenty times as great as the base of the stapes. Thus the tym- panic membrane concentrates its power upon an area only one-twentieth of its size, and this, increased by the shorter arm of the lever (of the incus), must give a force at least thirty times as great as that with which the handle of the malleus is moved at the umbo of the tympanum. Another reason why the stapes cannot move far is found in the firm- ness of the fibres of the membrana tympani, and of its attachment to the handle of the malleus ; extensive move- 214 Physiology of the Senses raent of the membrane is thus prevented. Lastly, where the membrane might move too freely, we have the action of the tensor tympani muscle coming into play. By the pull inwards of this muscle upon the handle of the malleus, the tension of the membrane is increased, and its extent of vibration correspondingly diminished. But this brings us to a consideration of the manner in which membranes respond to sonorous vibrations. Response of the Tympanic Membrane to Sound Waves. — The physical cause of the sensation of sound is the rapid vibration to and fro of the molecules of an elastic medium when these have been set in motion by a sudden shock. The particles, when disturbed, vibrate to and fro till they regain their former equilibrium. Such vibration may be transmitted from molecule to molecule through solids, liquids, or gases. Thus the arm of a tuning-fork, when set in vibration, causes an alternate condensation and rare- faction of the air in the space through which it moves. With each successive to -and -fro movement of the fork another alternation of change of density is set up, and this is propagated outwards in all directions from the fork as a centre. The direction of movement of the particles in a sound wave is not transverse to the direction in which the wave is moving, but in the same direction. Hence they are said to be longitudinal waves, as distinguished from the transverse movements characteristic of waves of light, or of waves moving on the surface of water. Such longitudinal waves can readily be set up in solids, as, for example, in a wooden rod by friction, and on account of the closeness to one another of the molecules in solids such vibrations are transmitted with great rapidity. But rods, strings, or mem- branes may be caused to vibrate transversely to their length or plane, as when a violin string is pulled aside by the bow, or a drum is beaten. If these vibrations be in quick Sound and Hearing 215 succession, they will give rise to sound waves in air. In this case it will be noticed that while the particles of the solid body are moving transversely to the length of the rod or string, or the plane of the membrane, their direction of vibration is still longitudinal in so far as the direction of the transmission of sound is concerned. The impulses given to the air by a vibrating string are of a complex type, for while it may vibrate as a whole, and give forth a series of waves, which combin- ing excite the sensation of a sound or tone, this fundamental tone is always modified by the presence of overtones produced by the simultaneous vibra- tion of segments of the string (Fig. 109). In the case of a rod or string these seg- ments are respectively a half, a third, a fourth, and so on, of the length of the whole rod or string, and the num- bers of vibrations given forth by these \ /' segments are respectively twice, three times, four times, and so on, that of the fundamental tone. In the case of A B plates or membranes, the number and FlG- I09- — Diagram of string vibrating so as to character of the overtones are more give forth its fundamental difficult to determine, being dependent tone (A>> and its first _ , upper partial tone or on the form and elasticity of the octave (B). plates, the manner in which they are set vibrating, and the number of vibrations. The smaller and the more tightly stretched a membrane is, the faster will be its rate of vibration and the higher the pitch of the sound thereby caused. On a large vibrating membrane the surface is, as it were, subdivided into many portions of vary- ing sizes, some small, some large, each vibrating at a rate peculiar to itself, and thus giving rise to a complicated set 2i6 Physiology of the Senses of aerial vibrations. Conversely, if the air is vibrating at any of the rates at which the membrane, or parts of it, may vibrate, the membrane will begin to vibrate in response. Suppose two violin strings, or two tuning-forks, are tuned to the same pitch and placed close to one another ; if one of these be set vibrating the other will also begin to vibrate at the same rate ; but strings or rods will not respond so readily as membranes to a variety of tones. Membranes respond more readily to aerial vibrations than plates do, because of the smaller mass of matter requiring to 'be moved, and the consequently greater flexibility of the surface. A drum-head will move freely to and fro under a blow which will cause almost no apparent result upon a thick plate. The application of these facts to the action of the tympanic membrane in hearing is not far to seek. In the first place, the membrane is small, very thin, its fibres are inelastic, and it is firmly but not evenly stretched in all its parts. From its thinness it can respond to aerial impulses of very faint kinetic energy. This receptivity we have seen may be interfered with by the accumulation of hardened wax upon the membrane. The peculiar arrangement of the fibres of the membrane makes it respond to sounds of widely-varying pitch. The fibres radiating from the umbo to their varying points of attachment in the tympanic groove constitute, as it were, a vast number of strings of varying lengths, each of which will respond most readily to its own particular tone. Again, the concentric circular fibres may be regarded as surround- ing a series of nearly circular discs of gradually increasing size, and therefore of different vibratile capacities. Further, von Helmholtz has shown that the shallow conical form of the membrane, the slight outward convexity of its fibres, renders it less liable to have a fundamental tone only, and Sound and Hearing 217 increases its receptivity for all varieties of sounds. It has been found that iF a handle be attached to a flat disc, and the disc be then curved like the tympanic membrane, it ceases to have a fundamental tone. This property of the drum-head is of paramount importance in hearing, as it leaves the ear free from the disadvantage of having all tones but one overburdened by a preponderating fundamental tone. Almost every ear will respond to tones having as low a frequency as 30 vibrations per second, while certain acute ears may hear tones caused by 40,000 vibrations per second. The receptivity of the tympanic membrane for sounds of high pitch, that is to say, sounds due to a large number of vibrations per second, is enhanced by the action of the tensor tympani muscle (Fig. 104). When this muscle contracts it pulls the handle of the malleus, and with it the tympanic membrane, inwards, and thus tightens the membrane just as a drum-head is made more tense when it is braced up. The fibres being tighter, their play is diminished, and they respond more readily to vibrations following in quick succession. On the other hand, by the action of the laxator tympani, the membrane becomes more flaccid and responds better to sounds of low pitch. It has been suggested that the power which many trained musicians have of recognising the absolute pitch of a note may depend to some extent upon the sense of muscular effort arising from varying degrees of contraction of the tensor tympani. In such cases long practice in the determination of the pitch of notes gives rise to such delicacy of judgment that there seems to be an intuitive and direct recognition of pitch, and not only may the pitch of a sound heard by the musician be named by him, but he may sing a note of any given pitch that he desires without the aid of tuning-fork or instrument. For the performance of this latter act, it is 218 Physiology of the Senses not unlikely that the parts unconsciously assume the neces- sary degree of tension before the sound fs uttered, just as we are apt to make involuntary contortions of the facial and other muscles when performing complicated or difficult actions. One important factor in the regulation of the tympanic membrane has still to be mentioned. If we strike the keys of a piano and hold them down so as to prevent the dampers touching the strings, the vibration of the strings will go on for a considerable time ; but when we release the keys, and the dampers touch the strings, the vibration stops. In the ear the handle of the malleus attached to the tympanic membrane acts as a damper. If the mem- brane went on vibrating after the sound wave had ceased, there might be interference with other succeeding sounds, but the duration of the vibration is cut short by the resist- ance offered by the chain of bones. The development of overtones in the membrane is likewise prevented, and the ear is rendered more acute in the discrimination of different sounds following one another in rapid succession, and each tone is heard pure, and not interfered with by those which have immediately preceded it. There is a further provision in the structure of the internal ear for differentiation of sounds, but this we will refer to afterwards. Transmission of Vibration by the Auditory Ossicles. — We have next to consider how auditory vibrations are conveyed to the internal ear. It has been experimentally determined that sound is mainly transmitted through the middle ear by the movement, as a whole, of the chain of bones. No doubt where these are absent, or have been rendered immovable by disease, a person may still be able to hear, but t"he acuteness of hearing will be largely interfered with. As to the nature of the movement of the bones there is a common consensus of opinion. It will be readily Sound and Hearing 2 1 9 understood that the movement of a solid body may be the resultant of many constituent elements. The earth rotates upon its axis whilst it moves round the sun. In a red-hot cannon ball projected through the air, the molecules of the metal are in a state of extremely rapid movement with reference to each other, as well as in transmission through space. In a tense string set into transverse vibration there must be a continual lengthening and shortening of the string, or in other words, a change in position of the mole- cules relatively to one another and in the direction of the length of the string as well as the transverse movement of the string as a whole. The longitudinal movement of the particles is invisible, the transverse movement is visible, to the naked eye. The former we call molecular, the latter molar movement. Probably there is some molecular move- ment of the ossicles of the ear, but the presence of joints must largely interfere with this, and the movement is mainly of the bones as a whole, that is to say, a molar move- ment, a movement that may be seen with the eye. While this is so, we must be careful to distinguish between the amount of movement of the bones and the length of the sound wave. The length of a sound wave is dependent not upon the amplitude of movement of the sounding body — that determines the intensity or loudness of the sound — but upon the number of vibrations made in a given time by the sounding body. In Fig. no, p. 220, A represents a long wave of small amplitude of movement, B short waves with greater amplitude. The length of the wave is measured by the interval between two successive points in like phase relatively to one another. Thus in A, we must move from a to c in order to get two particles in like condition of velocity and direction of movement, so we say that ac is the length of the wave. Now the distance through which a sound wave will pass in any medium in a 220 Physiology of the Senses given time depends upon the elasticity and density of the body in question. Through air, sound waves pass, on an average, at the rate of 1120 feet per second. If, then, a body makes a complete to-and-fro vibration only once each second, the first movement must have passed 1020 feet before the second begins, or in other words, the wave- length is 1 1 20 feet. If the body performs a complete vibration twice in a second the distance between two points of like condensation and rarefaction will only be one- half of 1 1 20 feet, or 560 feet. The more rapid the rate of vibration, the faster will wave succeed wave, and the shorter will the wave be. The ear can readily distinguish as a musical tone sounds due to vibrations following each A A A A A A A 11 " VJ/ W \J \J \J W V W FIG. no. — Diagram illustrating (I.) long waves of small amplitude, and (II.) short waves of greater proportional amplitude. other thirty times in a second. The wave-length in such a case would be 1120^30 = 37 feet approximately, while certain ears can hear a sound due to 40,000 vibrations per second, in which case the wave-length will be 1120 feet -j- 40,000, or approximately ^ of an inch. But in either case it will be seen that the bones of the ear cannot move through the " length of the wave," but rather that the. time of recurrence of like condition of condensation or rare- faction at the drum-head gives rise to our appreciation of differences of pitch. Regularly succeeding stimuli going to the auditory nerve at the rate of say thirty times a second will give rise to a sensation of a sound of low pitch, and if at the rate of say 4000, to a sensation of a sound of high pitch. The length of the wave is of importance in regulat- Sound and Hearing 221 ing the number of times per second the drum-head will vibrate, taking into account the rate of the transmission of sound waves through air ; but the breadth of the ear, and even of the whole head, may only form a very small part of the length of the wave. A tuning-fork bowed gently will give a sound of the same pitch as the same fork bowed strongly. In the one case we cannot see any movement in the limbs of the fork ; in the latter the sharp outline of the limbs is lost, and we can see at once that the limbs are in motion. Similarly in the ear. With weak sounds the drum-head hardly moves, and the ossicles seem to be at rest, but if the sound is loud, the drum-head and the bones may be seen in motion.1 With very loud sounds, when many molecules of air have been suddenly compressed into a small space, the pressure upon, and consequent move- ment of, the tympanic membrane is very great, and the force may even be so excessive as to cause rupture of the membrane, just as windows are sometimes shattered by a violent and consequently loud explosion. While in ordinary circumstances the tympanic mem- brane is usually thrown into vibration through the medium of the air in the external meatus, it should be borne in mind that it may be set in motion also by transmission of vibrations through the bones of the skull. If a tuning-fork is struck, and its handle pressed against the teeth, a molecular movement is transmitted to the membrane with such energy as to set the membrane and ossicles into visible molar movement. We can illustrate 1 A preparation can be made of the ear of a dead cat. The middle ear is laid open by removing a small portion of its wall. After lightly dusting the interior with lycopodium powder, it is strongly illuminated and examined with a microscope of moderate power. When the vibrations of an organ pipe, sounding loudly, are directed into the external ear, little brilliant specks of lycopodium powder may be seen to vibrate. 222 Physiology of the Senses this by placing a number of marbles in a row, and touching one another. If a smart tap be given to the marble at one end of the row, it will not apparently move, nor will the intervening members of the series, but the last marble of the row will fly off as if directly struck. The energy of the blow is, in this case, transmitted through the molecules of the marbles, and is sufficient to give rise to visible move- ment in the last member of the series. So the movement transmitted through the bones of the skull gives rise to free movement of the tympanic membrane, and through it to the internal ear. Trial, however, will show that the tym- panic membrane responds better to the vibrations of the air in the meatus than to those transmitted through the head. If a tuning-fork be struck, and its handle held between the teeth till the sound has apparently ceased, and if then the fork be held opposite the ear, the sound will be distinctly heard again. We may attribute this to the greater mobility of the molecules of air in the meatus than that of the molecules of the bones of the head. They move more freely to and fro, and under a feebler stimulus, than the molecules of the bones, and thus the membrane responds more readily to the tuning-fork held to the ear. Still, although both membranes be absent, the ear is quite capable of hearing and of distinguishing musical sounds by the direct stimulation of the internal ear, and its apprecia- tion of pitch cannot be affected, inasmuch as this is due to the physical fact of a recurrence of stimuli at definite intervals of time. The intensity of the sound will, how- ever, be diminished, because, as we have seen, the arrange- ment of membrane and ossicles gives a mechanical advan- tage in the way of increased power. Sound and Hearing 223 THE INTERNAL EAR We have already said that the internal ear consists of a closed sac formed by an invagihation of part of the skin at a very early period of life, and that the nerve of hearing ends in this sac. We have now to consider the form of the internal ear, the mode of ending of the auditory in which its structure is adapted to the function of hearing. And, in the first place, let it be noted that modern research tends to confirm a conjecture made long ago that the front part of the internal ear, the cochlea, has to perform an entirely different function from the posterior part. In correspondence with this, the auditory nerve has been shown to consist of two nerves (Fig. 103,^, /) which, arising in different parts of the brain, are united by connective tissue in the greater part of their course, FIG. in. — Right bony labyrinth viewed from the outside (X 2^, and natural size). The more spongy material of the petrous bone has been separated from the dense bony wall of the labyrinth, i, The vestibule ; 2, fenestra ovalis or oval window ; 3, superior semicircular canal ; 4, horizontal or external semi- circular canal ; 5, posterior semicircular canal ; * * ampullae or dilatations of semicircular canals ; 6, first coil of the cochlea ; 7, second coil ; 8, apex ; 9, fenestra rotunda or round window. (Sommerring.) but separate again as they approach their termination, and end in organs which differ widely in appearance from each other. The posterior portion of the sac is contained in the bony cavity known as the vestibule and semicircular canals. 224 Physiology of the Senses We may imagine the canals as having been cut off from the main body of the sac by the meeting and agglutination of opposite parts of the original cavity, just as if, were we to press together between thumb and finger the opposite sides of a bag near one of its corners, we would form a canal or passage communicating at each end with the main cavity of the bag (Fig. 1 1 2). This main cavity in the ear is known as the utricle (Fig. 113); it is oblong in shape, being about one- fourth of an inch long, and communicates behind and above with three semicircular canals (Fig. n i, 3, 4, 5) which lie respectively in three planes, one horizontal and two vertical, and all exactly at right angles to each other like three FIG. 112. — Diagrammatic representation of the manner in which the semicircular canals are formed from a primary cavity. (See text.) adjacent sides of a cube. From the direction in which the curves are inclined, the canals are named respectively the horizontal or external, the antero- posterior, or simply the posterior and the transverse or superior canals. Each canal has one of its openings into the utricle dilated to form what is known as an ampulla (Fig. in), the other end passing into the utricle without enlargement, and the undilated ends of the canals in the vertical planes unite with one another before passing to the utricle, so that there are only five openings for the canals into the utricle, three of which are provided with ampullae. The utricle lies in the vestibule. Below, and in close apposition to, the utricle, and, like it, contained in the Sound and Hearing 225 vestibule, we have the saccule (Fig. 113), a smaller and more rounded space than the utricle. These two cavities are formed by a constriction of the primary vesicle, and even in adult life are in connection with each other by a long narrow tube of a Y shape, the ductus endolymphaticus (Fig. 1 1 3), one part of which actually penetrates through the bone into the cavity of the skull, and lies enclosed by the membranes surrounding the brain. The saccule, by a narrow tube, the canalis reuniens (Fig. 113), communicates with the long finger-like projection, the canal of the cochlea, which is packed away in small space by being wound two and a half times round a central supporting pillar of bone, the modiolus(¥\g. 117). The auditory nerve, enter- ing the bone containing the internal ear by a passage called the internal auditory meatus, divides, as it enters the bony labyrinth, into two main divisions, one going to the cochlea, and the other to the vestibular part of the membranous labyrinth, the latter branch quickly dividing further so as to supply a terminal branch to the utricle, the saccule, and the ampullae of the semicircular canals, and to these parts alone. The membranous labyrinth has for its outer coating a layer of connective tissue from which numerous processes pass to the fibrous lining of the bone. The spaces between the processes, similar to other lymph spaces throughout the body, are lined with flat cells and filled with a somewhat viscous fluid. The connective tissue is homo- logous with the true skin, and like it contains blood-vessels, Q FIG. 113. — Membranous labyrinth (diagrammatic), c, Cochlea ; s, sac- cule united by/, the ductus endolym- phaticus, with «, the utricle, arising from which are seen the three semi- circular canals. 226 Physiology of the Senses The inner lining of the sac, except where the nerves end, consists of a single layer of flattened cells. In one portion of the utricle and of the saccule lies a small oval spot, or macula, and in the ampulla of each canal a ridge or crisiq Sound and Hearing 227 which, since they contain the termination of the vestibular nerves, are known respectively as a macula or crista acustica. Over these the epithelium is stratified, being mainly made up of thread-like columnar cells (Fig. 115), having a well- marked nucleus, and supporting another set of nucleated cylindrical cells, whose-free surfaces bear bunches of stiff rod-like hairs which are often adherent one to another, and are known as the auditory hairs. Some observers have described the hairs as pass- ing through a membrane similar to that found in the cochlea (p. 236) ; but this has been disputed. The terminal twigs of the auditory nerve, passing through the con- FIG. 115.— Epithelial nective tissue which forms the main sub- cells from macula stance of the prominence or ridge, lose acustica of the utricle. their outer sheaths and pass as naked axis- cylinders into the epithelium, where their mode of termina- tion is not definitely known. Some suppose that they end in the cells, others that they simply surround them with a nest of fine fibrils ; but, from analogy with the other sense organs, we may conjecture that they are at least stimulated by the agitation of the hair- cells. The free ends of the auditory hairs are embedded in a soft mucous mate- rial, the cupula^ in which are often found F from ^thf^puia sma11 cr>7stals consisting largely of carbonate above the human oflime, called otoconia, or otoliths(¥\g. 1 1 6). macula acustica. The function of this covering is unknown, though it has been supposed to act as a damper to the vibration of the auditory hairs. It may possibly be driven mechanically against the points of the hairs by vibrations of sound, and thus increase the sensitiveness of the hairs to such vibrations, 228 Physiology of the Senses The Cochlea. — We come now to consider the struc- ture of the cochlea (Gr. cochlias^ a snail with spiral shell), which is a tubular cavity coiled in a spiral manner round a central pillar called the modiohts. The part of the mem- branous labyrinth which it contains is much smaller in cross section than the bony space, and is known as the canalis cochlearis. It is fixed in the whole of its course, except at its closed end, to either side of the cochlea, having a broad surface of attachment on the outside, but a very narrow one towards the median column. Indeed, we find here that the cochlear canal is only attached on its inner aspect to the free edge of a shelf which winds round the central pillar, projects outwards into the lumen of the cochlea, and is known FIG. ,i7.-The osseous cochlea divided as the lamina SPiralis °SS™, through the middle, X 5. i, Central or Spiral plate of bone. It canal of the modiolus in which lies the ^ . f d ^ cochlear nerve ; 2, the spiral osseous lamina; 3, scala tympani ; 4, scala plate of bone, between the vestibuli ; 5, spongy bone of modiolus surfaces of which the nerVeS near the spiral canal, 8. (Arnold.) pass out from the central column to enter the cochlear canal. In a section made transversely through one of the whorls of the cochlea, we see then three spaces represented in Fig. 118. The upper space, containing perilymph, is in connection, at its beginning, with the vestibule, and, as it winds round towards the apex of the cochlea, it is known as the stair- way from the vestibule or scala vestibuli. At the summit of the cone it bends round the closed end of the cochlear canal and the free hook-like end or hamulus of the lamina spiralis, by a little passage called the helicotrema, and communicates with a descending space which, winding Sound and Hearing 229 round the modiolus, ends at the fenestra rotunda, whose membrane closes the opening into the middle ear. This lower space is known as the scala tympani. The two scalas are lined with a connective tissue membrane which is thickened on the outer wall to form the spiral ligament, first described by Bowman, and the free surface of the membrane is covered with a single layer of flattened cells. The scalae being in connection with each other at the top FIG. 118.— Section through one of the coils of the cochlea (diagrammatic). SV, Scala vestibuli ; ST, scala tympani ; CC, canal of the cochlea ; /so, lamina spiralis ossea, or spiral plate of bone ; Us, limbus of the spiral lamina ; R, Reissner's membrane ; ss, spiral sulcus or groove ; t, tectorial membrane ; CO, organ of Corti ; b, basilar membrane ; Isp, spiral ligament ; nc, cochlear nerve ; gs} spiral ganglion in course of cochlear nerve. (After Henle. ) of the whorl, and being filled with perilymph, the pressure of the fluid in the two spaces must be the same when the ear is at rest. If, by the movement of the stapes, the pressure of the fluid in the vestibule be increased or diminished, there must be a corresponding change of pressure transmitted from the scala vestibuli to the scala tympani, and this may be effected either directly through the cochlear canal or through the helicotrema. The fluids of the ear being practically incompressible there must be a 230 Physiology of the Senses corresponding movement of the membrane closing the fenestra rotunda. Upon the upper surface of the spiral bony shelf, and near its free border, is a thickening of the connective tissue known as the limbus. This thins away as it covers the free edge of the shelf, and a groove is formed — the sulcus spiralis (Fig. 1 1 8) — whose free borders are known respec- tively as the vestibular and tympanic lips. The Cochlear Canal. — In cross section, the canal of the cochlea is roughly triangular in shape, the apex being attached to the spiral plate of bone, the base to the outer wall of the cochlea. That part of the wall of the canal which looks towards the scala vestibuli arises from the upper surface of the spiral shelf a little nearer the modiolus than the limbus, and stretches as a thin fibrous membrane, known as Reissner's membrane, to the outer wall. It is lined on its vestibular side by flattened cells, while the internal surface is clothed with more cubical cells, some of which have probably a secretory function. The wall of the cochlear canal, which takes part in the formation of the scala tympani, stretches from the tympanic lip of the spiral lamina to the spiral ligament, and is known as the lamina spiralis membranacea^ or basilar membrane. It is indistinctly fibrous towards its inner attachment, but in its outer two-thirds shows a radial fibrillation as of rod- like fibres embedded in a homogeneous matrix. This part of the structure is, as we shall see, probably of considerable importance in the appreciation of the pitch of sounds. The tympanic surface is lined with cells, often of a spindle shape, which lie transversely to the fibres above them, and, at one part immediately below the organ of Corti about to be described, we find a small blood-vessel, the vas spirale, which ensures a good blood supply to the superjacent structures. Sound and Hearing 231 The Organ of Corti. — The epithelium upon the upper, or, with reference to its position in the head, anterior surface of 7 A 9 I1\ F FIG. 119. — Cross section of the human cochlear duct at the junction of the first and second turns of the cochlea, X 100. i, Outer wall (part of the spiral ligament) reaching from b to c ; 2, vestibular wall, or Reissner's membrane, from a to c ; tympanic wall from a to b ; 3, lamina of bone ; 4, its vestibular lip ; 5, its tympanic lip ; 6, nerves of hearing passing to epithelium at 7 ; 8, internal spiral groove with flattened epithelium ; 9, basilar membrane ; 10, its tympanic covering ; n, basilar crest of spiral ligament ; 12, prominence of spiral ligament with blood-vessel ; between n and 12, the external spiral groove ; 13, vascular layer ; 14, spiral papilla (epithelium of Corti's organ) ; near 14, the outer hair-cells and Deiter's cells; further inwards the rods of Corti covering the tunnel ; internal to this the inner row of hair-cells ; 15, the tectorial membrane. (After Retzius.) the basilar membrane is of a highly specialised type, and more especially that part which rests upon the inner half of the membrane. This part is commonly known as the organ 232 Physiology of the Senses of Corti, from the Italian Marquis of that name who first gave a detailed description of it. When we examine sections made transversely to the length of the canal, we find a peculiar structure resting upon the basilar membrane immediately adjoining its inner line of attachment. This consists of a set of elongated rod-like cells arranged in two rows throughout almost the whole length of the cochlear canal, and known as the outer and inner rods of Corti. These rod-cells, rising from the membrane, meet at their upper ends like the beams of a sloping roof, and, together with the membrane, enclose'a space called the tunnel. The individual rods have a cylin- drical form and an expanded base, by which they are fixed to the basilar membrane. The e upper ends of the rods are FIG. 120.— Inner and outer rods of Corti from the cochlea of a guinea- enlarged, but flattened at the pig, X 275. A, Inner rod-cell ; B, ^ ^^ ^ ^ • t t outer rod-cell. In both are seen — i, the foot piece; 2, the body ; and, with adjoining rods, and the 3, upper end of rods ; 4 nucleus - h d h th j and protoplasm. (Schwalbe.) outer aspect a socket into which fit the rounded heads of the outer row of rods. From the head of each rod there projects outwards a flattened process, those of the inner row overlapping those of the outer. The inner rods are about a half more numerous than the outer, so that two outer rods fit into three of the inner row. At the base of each rod we find a nucleus and granular protoplasmic material, while the main substance of the rod exhibits no structure, or merely a faint longitudinal striation. The rods being placed in line, and all the head-plates being similar in size and appearance, they present, when seen from above, a remarkable resemblance to the key-board of a piano. Sound and Hearing 233 Fibres of the auditory nerve pass between the rods and across the tunnel, which, during life, contains also a colour- less jelly-like intercellular substance (Fig. 121). ~ -1 FIG. 121. — Surface view of the spiral papilla of Corti's organ from the topmost coil of a rabbit's cochlea, from the inner hair-cells to the cells of Deiter. (After Retzius.) Highly magnified, i, Inner row of hair-cells ; 2, boundary line of their surface ; 3, cuticle of the inner hair-cells, each showing eight hairs ; to the left an extra inner cell is present ; 4, flattened tops of the inner rods of Corti ; 5, outer border of these plates ; these completely cover the tops of the outer row of rods, seen between 6 and 7; at 6 is seen the inner border line of attachment of the heads of the outer rods. From the tops of the outer rods are seen at j the processes to the phalange, narrow at 8, and widening at 9 to form part of the lamina reticularis. 10, Phalanga? of the first row. n, Phalangae of the second row. 10-12 are the cuticular end plates of the three rows of Deiter's cells. In the interspaces between these appear three rows of outer hair-cells, each showing eight hairs, arranged in horse-shoe shape, projecting from their free cuticular surface. The Inner Hair-Cells. — Just to the inner side of the rods of Corti we find a row of columnar cells whose free 234 Physiology of the Senses surface is on a level with the head of the inner rods upon which they rest. Each of these columnar cells has project- FIG. 122. — Radial section through the tympanic wall of the middle of the cochlear duct of the guinea-pig, X 212. i and 2, Upper and lower plates of the osseous spiral lamina ; 3, spiral ganglion ; 4, spiral bundle of medullated nerve fibres ; 5, medullated nerve fibres radiating outwards between the bony plates of the spiral lamina ; 6, thin connective tissue lining bone (periosteum) ; 7, limbus of the spiral lamina ; 8, its vestibular lip ; 9, its tympanic lip, through which at 10 the nerve fibres, losing their medullary sheath, pass to the epithelium ; n, beginning of Reissner's membrane ; 12, union of tympanic lip with basilar membrane ; 13, nucleated transparent layer of the basilar membrane ; 14, layer of basilar fibres ; 15, cellular lining of basilar mem- brane ; 16, epithelium of internal spiral groove ; 17, inner supporting cells, below which the nerves emerge ; 18, inner hair-cells ; 19, inner rod of Corti, a, nucleus and protoplasm ; 20, outer rod of Corti with, b, its nucleus and pro- toplasm ; c, cross section of spiral bundle of nerve fibres winding up with the tunnel ; from it the nerve fibres, d, pass outwards between the outer rods of Corti to the outer hair-cells ; 21, outer hair-cells in three rows alternating with phalangar processes, 22, of Deiter's cells, 23 ; 24, supporting fibres of Deiter's cells ; 25, cells of Hensen ; 26, cells of Claudius ; 27, membrana tectoria ; 28, its marginal thickening. (Schwalbe.) ing from its free surface from fifteen to twenty short stiff hairs arranged in a crescentic line, whose convexity faces outwards. The attached ends of the hair-cells are conical Sound and Hearing 235 in shape, and do not come down to the basilar membrane, but are connected with, or closely invested by, terminal fibrils of the auditory nerve. There may also be seen around and below the lower ends of the hair-cells a number of nuclei. These belong to elongated filamentous cells, which, arising from the beginning of the basilar membrane, pass to the surface between, and to the inside of, the hair- cells, and, in all probability, act like the rods of Corti as supporting structures. From the inner row of hair-cells epithelial cells, at first columnar, then more cubical or even flattened, line the spiral groove already referred to, but the overhanging part of the vestibular lip of the limbus is devoid of epithelium, and is broken up by slight radial markings into a set of projections known as the auditory teeth. Outer Hair-Cells. — To the outer side of the rods of Corti we find rows of hair-cells and supporting cells similar in many ways to the row found to the inside of the rods. In the human ear there are usually four rows of hair-cells, but there may be only three, or as many as five, rows in certain parts of the canal. In the ears of lower mammals there are seldom so many rows as in man. The hair-cells of the outer row are likewise columnar, have short stiff hairs arranged in a semicircular or horse- shoe shape— convexity outwards — on their free surface, a nucleus surrounded by granular protoplasm, and nearer their free border a dark pigmented spot known as Hcnserts spot. The lower ends of the hair-cells do not pass down to the basilar membrane, but, like the inner row of hair-cells, are in contact with the terminal fibrils of the auditory nerve. Closely apposed to the outside of each of the hair-cells in the outer rows is a supporting structure, known as Deiter^s cell (see Fig. 122), which, arising by a thicker nucleated part from the basilar membrane, gradually becomes nar- rower and passes, as a small cylindrical process, to the free 236 ' Physiology of the Senses surface. Here the Deiterian cells are fixed to fiddle-shaped plates — phalange — which, uniting with adjoining plates, and with the processes from the heads of the rods of Corti, form a fenestrated or reticulated 'membrane •, in the meshes of which lie the free ends of the hair-cells. Each hair-cell is thus fixed to and supported by a structure, which is itself inserted at either end into a membrane, and thus the com- ponent cells are firmly held in their respective places, and we can see that any movement of the basilar membrane must be at once communicated to the hair-cells through the medium of Deiter's cells. Outside of the rows of hair-cells we find, for a short dis- tance, a row of columnar cells, devoid of hairs, and having no direct connection with the auditory nerve. They are known as Henserfs cells, and they soon merge into a layer of cubical cells, the cells of Claudius^ which cover the outer third of the basilar membrane, and are continued over the spiral ligament and that part of the cochlear canal which is in contact with the outer cochlear wall. The spiral ligament into which the basilar membrane is fixed, consists in the main of connective tissue, but spindle- shaped cells have been described as existing in it, which, as first suggested by Bowman, are supposed to be muscular, and whose function would be to tighten the basilar mem- brane, and adapt it for variations of pitch. The spiral ligament is vascular, and at one part a slight elevation (vas prominens) is .made by a vein (Fig. 119). It will be seen that the neuro-epithelium of the cochlea resembles, in many respects, that found in the vestibular part of the internal ear. This likeness is further increased by the fact that we find, lying in the cochlear canal, fixed at one end to the vestibular lip of the limbus, and at the other free or attached to the outer part of the organ of Sound and Hearing 237 Corti, a thickish layer of fibrous tissue known as the mem- brana tectoria. This may, as conjectured in the case of the cupula, act as a damper when resting on the hair-cells, but its action is not known. Innervation of the Cochlea. — The cochlea is supplied by a branch of the auditory nerve. The modiolus or cen- tral column, round which the cochlea is coiled, is hollowed out in a conical fashion, the space being filled by the coch- lear nerve, which, comparatively thick at first, soon lessens in diameter by giving off numerous branches which pass out into the bony spiral shelf. Before reaching their ulti- mate destination, however, the fibres pass into a mass of ganglionic nerve-cells of a spindle or bi-polar form, which form a continuous spiral from the base to nearly the apex of the cochlea, known as the spiral ganglion (Fig. 122). From this the fibres emerge in bundles which coalesce to form finer bundles. These passing radially outwards, be- tween the opposing surfaces of the spiral lamina, emerge in little furrows or canals at the tympanic lip, called foramina nervina, and, losing here their primitive sheath and white medullary substance, pass as bare axis-cylinders into the neuro-epithelium of Corti's organ. The nerve fibres do not seem to pass directly after emerging from the bony plate to the hair-cells opposite. They seem rather to bend round and run in the direction of the cochlear spiral, some below the inner row of hair- cells, some, after entering the tunnel, through interstices between the rods of Corti, and some in spaces between each row of the Belter's cells supporting the outer row of hair- cells. There are thus an inner spiral strand, a spiral strand of the tunnel, and three or four outer spiral strands. From these spirals are given off the ultimate fibrils which proceed to the hair-cells. Whether they pass into these, or simply into contact with them, is not definitely known. We may, 238 Physiology of the Senses however, feel assured, both from analogy and from careful study of the structure, that the hair-cells are the true ter- minal organs of the auditory nerve, that they alone can respond to auditory vibrations, and set up sensory impulses in the auditory nerve, and that the other cells of Corti's organ are merely accessory in function. In birds, for instance, the cochlea is very rudimentary, consisting of a small protuberance from the saccule, and containing only hair-cells on a basilar membrane and no rods of Corti. It may seem strange that in birds, even in the sweetest song- sters, the part of the ear which seems specially devoted to the appreciation of musical tones should be ill developed j but it must be remembered that the quality and variety of tones of the bird's song are vastly inferior to those of the human voice, nor has the brain of the bird the development necessary for the due recognition of the variety of sounds which the human brain can differentiate. In the human ear itself, the structure of Corti's organ varies as we pass from the beginning to the end of the canal. At first, where it unites with the canalis reuniens (p. 225), it is lined with ordinary epithelium. Then the organ of Corti has at first only three rows of hair-cells ; farther on, four rows appear, and in some ears five. At the closed end of the canal, the neuro-epithelium is again awanting, and gives place to a simple squamous epithelium. Observations are still required with regard to the com- parative powers of ears as regards the appreciation of vary- ing sounds according to the number pf hair-cells which may be present. While the general principle of formation of Corti's organ remains the same throughout the whole length of the cochlea, the grouping of the supporting cells, and more especially those of Hensen, gives different appearances at different levels of the spiral. It is also noteworthy that the basilar membrane varies in breadth, not, as was at one Sound and Hearing ' 239 time supposed, narrowing from base to apex, but actually in- creasing from .2 1 mm. (j-J-g inch) to .36 mm. (nearly y-^-g- inch) (Retzius) in breadth as it ascends. Thus, if we regard its radial fibres as corresponding to the strings of >a musical instrument, such as the harp, those fibres which lie at the base of the cochlea, and consequently nearest the vestibule, would compare with the short strings of the harp, which vibrate rapidly, and give forth sounds of high pitch, while those at the apex of the cochlea correspond to the long strings which emit a bass note. If, as has been supposed, this analogy is not a merely fanciful one, it is manifest that we have in this arrangement the greatest mechanical advan- tage, tones of short wave-length obtaining immediate response, while those of greater wave-length must travel FIG. 123.— Diagram illustrating change in breadth of the basilar membrane from base to apex of cochlea ; the length of the diagram is about twice, the breadth about ten times, the actual dimensions ; the numbers in the diagram indicate in millimetres the size of the structure in the ear, not the lengths of the lines. farther. The basilar membrane being, according to Retzius, about 35 mm. (ifth inch) in length, the accompanying diagram (Fig. 123) represents on an enlarged scale the com- parative breadth of the membrane in different parts in relationship to each other, and to the length of the canal. The actual difference in the length of the fibres is, as will be seen," very little, and it should further be noticed that the distinct fibrillation of the membrane is well marked only in the outer side of the membrane, between the outer rows of hair-cells and the attachment of the membrane to the spiral ligament. If this part alone be -considered, we find that the ratio is sojnewhat altered — namely, from .075 mm. at the base to . 1 26 mm. at the apex, or nearly i : 2 instead of 3:5. The difference in absolute size may seem very little, 240 * Physiology of the Senses but we must always bear in mind the exceeding minuteness of all the parts involved, and the extreme delicacy with which so small an organ must be constructed in order to give such complex and varied results as does the human ear. The presence of what seem to be contractile cells in the spiral ligament lends colour to the supposition that, in the length and tension of the fibres of the basilar mem- brane, we are to look for the mechanism for the appreciation of pitch. We have said that possibly, in the cultivated musical ear, the training of the muscles attached to the drum-head, or rather the recognition of the muscular sensa- tion caused by varying degrees of contraction of these muscles, may play a large part. It may now be added that this sensation may be strengthened by the feeling of tension in the spiral ligament ; but at present this is merely a conjecture. AUDITORY SENSATIONS Physiological Characters of Sounds. — We have already referred briefly to the physical causation of sound, and we shall now consider how the physiological variations arise in connection therewith. When we seek to analyse the effect produced in consciousness by the stimulation of the auditory mechanism, we find that all sounds may be roughly divided, in the first place, into such as we designate noises, and those recognised as musical tones. The sounds of a peal of thunder, of the rending of silk, of the creaking of a door on dry hinges — these we call noises ; but when a tuning- fork vibrates, or a note on the piano is sounded, we call the effect produced upon the ear musical. The difference, how- ever, between a noise and a musical sound is not of a hard and fast kind. One may merge insensibly into the other. The tuning of musical instruments by an orchestra gives us Sound and Hearing 241 a noise as result, but the noise is made up of musical tones, and many sounds usually dismissed as noises, such as street calls, the barking of dogs, or the blast of a fog-horn, contain a distinctly musical element. When aerial vibra- tions agitate the ear in regular recurrence, when equal periods of time elapse between each stimulation, the sound produced is musical ; but in the example mentioned above, of the sound produced when an orchestra tunes its instru- ments, the musical tones from the different players come at irregular intervals, and at rates which interfere with one another in such a way as to produce a harsh or unmusical sound. On the other hand, sounds professedly musical are sometimes noises of the most disagreeable nature. As a combination of musical tones may produce a noise, we will best arrive at a clear comprehension of auditory sensations in general by the study in the main of musical sounds. Apart from the emotional feelings which may be aroused by music, there are certain sensations produced in the mind on hearing a musical tone. These sensations may be divided under three heads — first, of pitch ; second, of intensity ; and third, a sensation of a special quality of the sound, dependent upon whether it is one simple sound, or a combination of simple sounds. In practice, we seldom hear simple musical tones, such as are produced by a tuning-fork. The sounds produced by such musical instru- ments as the piano, violin, or flute, are not simple tones, but sounds in which many simple tones are blended into one so as to give a sound with a special quality, timbre, or klang, by which we can recognise the kind of instrument that has given it forth. But, given the pitch, intensity, and quality of a sound, we can, with proper instruments, reproduce any variety of tone we please. We shall con- sider, then, in the first place, the nature of pitch and of intensity or loudness, and then how tones of varying pitch R 242 Physiology of the Senses and intensity combine to give rise to a sensation of quality in a musical tone. i. Pitch. — The pitch of a tone depends upon the fre- quency of the vibrations in a given time ; or, to put it in another way, since the wave-length is shorter in direct pro- portion to the rapidity of recurrence, the pitch depends upon the length of the waves which go to produce the sound. If the vibrations come too slowly or too rapidly, no musical sound is perceived, and while ears may hear musical tones produced by vibrations at rates varying from about 30 to 40,000 per second, the range of the tones employed in music lies between 30 and 4000 per second. The fact that pitch depends upon frequency of vibration can be easily demonstrated by means of an instrument called the syren. This, in its simplest form, is a thin metal plate revolving upon an axle at a rate which can be exactly regulated. The plate is perforated by a set of holes at equal distances from the axle and from one another. The wheel is first caused to rotate slowly, and a current of air is blown against the plate, so that it will pass through the holes when they pass a certain point. At first a series of puffs is heard, but, as the speed of rotation is gradually increased, the puffs begin to coalesce, and when they recur at from 20 to 30 times a second, a low buzzing or droning sound is heard. The faster the plate revolves, the more numerous the puffs become, and the higher will be the pitch, until at last the sound grows faint and ceases to be audible. When the pitch of a sound is very high, the effect produced upon the listener is unpleasant. It is as if a thin metallic blade or needle were piercing the ears, or it may be compared to the shimmering effect of sunlight re- flected by the ripplets on the surface of water agitated by a light breeze. If the plate be made to rotate quickly and at constant speed, the pitch of the note will remain the same. Sound and Hearing 243 Von Helmholtz has devised a double syren, with which many interesting experiments can be performed as to the nature FIG. 124. — Double Syren of von Helmholtz. #o> a\i Brass wind-chests com- municating by tubes, g§, g\, with bellows ; the opposite ends of the cylinders are closed by brass plates perforated with holes corresponding to those seen in the disk, CQ ; the disks, CQ, ci, rotate on a common axis, k, provided with a screw for the counting apparatus, which is omitted here. The upper cylinder, a, can be rotated on a vertical axis in either direction by toothed wheel, e, with handle, d\ the four rows of holes may be opened or shut by means of studs, /, ;'; there are 8, 10, 12, and 18 holes respectively in the four rows of holes in the lower disk, and 9, 12, 13, and 16 in the upper (not seen in diagram). of pitch. It consists (Fig. 124) of two boxes, supplied by bellows with air, which, emerging through the lids of the 244 Physiology of the Senses boxes by holes, the number of which can be varied, causes a plate close to, and in a parallel plane with, the lid of each box to rotate. The rotation of the parallel plates allows the air to escape through several series of holes in them, just as in the simple syren. The beauty of the mechanism lies in the power it gives us of regulating exactly the num- ber of impulses per second, of reading off the number upon a dial, and of permitting us to note the effects produced when the two syrens are emitting tones of different pitch. It is thus most valuable in studying concords, discords, and beats, the nature of which will be described shortly. One point which invariably arrests the attention when the syren is heard for the first time is the peculiar effect of the gradual rise in pitch as the velocity of rotation is accelerated. We may say that at one moment it is giving forth many im- pulses, say, 200 per second ; at another a different num- ber, say, 201 ; but the change from 200 to 201 is through an infinite fractional series ; and so vrith regard to the sound ; it does not rise by leaps and bounds, but glides up in con- tinuous transition. Just as the colours of the spectrum vary through an infinite series, in passing from one colour to another, so do the sounds in changing from one pitch to another. The same effect can be produced on the violin by sliding the finger up the string while it is being bowed. And, further, as has been mentioned with regard to per- ception of colour, as some eyes are insensible to the red, and others to the violet end of the spectrum, so some ears are insensitive to sounds of low pitch, others to those of high pitch. As might naturally be expected, the sensibility to pitch varies more in the higher than in the lower parts of the scale, and we find people who suppose their powers of hearing to be perfectly normal, who yet fail to hear sounds due to more than 6000 vibrations. Test of power in this respect may be made by means of a set of short- Sound and Hearing 245 steel cylinders, made by Konig, which, when suspended by threads to a wooden frame, and struck with a metallic instrument, emit tones to upwards of 40,000 vibrations per second. The same result may be attained by using short- limbed or heavy tuning-forks. Within the range of musical pitch, too, we find that people vary much in their capability of distinguishing a tone of one pitch from another nearly the same. This likewise holds good in respect of colour. Orientals distinguish many shades of colours, which seem the same to us. While most people can detect a difference of a semitone in two notes sounding together when of medium pitch, some acute ears can detect as small a difference as -g^th of a semitone. It becomes more and more difficult to detect the difference as we pass to the upper or lower limits of hearing — a fact one may readily prove for oneself by striking adjoining keys, now in the centre, now at either end of the key-board of a piano. We have already indicated that the power of detecting variations in pitch can be increased by exercise and trailing, and have suggested a possible explanation as to how this is GO. On the other hand, there are some people who are unable to discriminate more than a very few tones, and who find it utterly impossible to sing any complicated tune. The pitch of the ordinary human voice in singing, it may be mentioned in passing, may be as low as faj (87 vibrations per second), or as high in a good soprano as so!4 (768 vibrations per second) ; or, in other words, it is comprised within a range of a little more than three octaves. There have been a few exceptional singers who have been able to sing pure musical notes be- yond these limits. Thus Gaspard Forster, a basso, passed from fa-! (42 vibrations) to Ia3 (435 vibrations); it is said that Nilsson, in // Flauto Magico, can take fa5 (1365 vibra- tions) ; and Mozart states that in Parma, in 1770, a soprano, 246 Physiology of the Senses Lucrezia Ajugari, ranged from so!2 (192 vibrations) to dop (2048 vibrations). The latter is the most highly pitched voice in musical literature, an octave and a half above the highest ordinary soprano. The extreme range of the human voice, then, taking into account the extraordinary voices above alluded to, is from fa-j (42 vibrations) to do6 (2048 vibrations), or about six octaves, while the range of the human ear for musical tones is from do — 1 (32 vibra- tions) to do10 (nearly 40,000 vibrations), or about eleven octaves. 2. Intensity or Loudness. — The second character of a musical tone which we notice is its intensity or loud- ness. This varies with the amplitude of vibration of the sounding body. Thus a tuning-fork bowed gently will give out a faint sound, while the same fork bowed strongly will give a note of the same pitch as the former, but sounding much louder. , In the case where the particles of the wave move at right angles to the direction in which the wave is advancing, as, for instance, a wave on the surface of water, one can readily understand what is meant by the height or amplitude of the wave. But this is not so easy in connection with a wave of sound where the particles are moving in the same direction as the wave, and we are apt to confuse the ampli- tude with the length of the wave, wh;ch, as we have seen, is invariable in any given medium for any given note, and determines pitch, not intensity. We can probably realise the meaning of amplitude best iri connection with sound waves by thinking of what happens when a large tuning- fork is vibrating feebly or strongly. In the one case, the excursion of the limbs is so small that, to the unaided eye, the fork seems to be motionless ; in the other, there is a perceptible movement through space, and though the pitch of the note remains the same, it has a louder, stronger Sound and Hearing 247 effect upon the ear. The fork makes exactly the same number of vibrations in each case, but in the latter its limbs move through a greater distance. Hence more molecules of air must at one moment be crowded into a ^iven space, at another there must be a more complete rarefaction of the air. There must then be a greater difference in the degree of pressure upon the drum-head of the ear; at one time a greater increase, at the next a greater diminution. Corresponding to this, there will be greater movement of the tympanic ossicles, and more variation in the pressure on the internal ear, and disturb- ance of the nervous arrangements. The contrast of loud and faint sounds can be readily made by holding to the ear a vibrating tuning-fork, and turning it round between finger and thumb, now this way, now that. It will be found — and this bears out the statement just made as to amplitude — that the sound is loudest when the plane in which the limbs are vibrating is at right angles to the side of the head, for here the air is disturbed with the greatest energy. The same experiment also shows the gradual transition in intensities just as in the case of pitch. The more the energy of vibration, or, in other words, the greater the number of molecules packed into a given space in a given time, the greater will be the loudness — a pheno- menon comparable to the sensation of varying brightness of light. 3. Quality, Timbre, Klang. — The quality of a musical sound enables us, after a due amount of training, to know, from the effect produced upon the ear, what is the instru- ment by which the sound has been produced. We readily distinguish, for example, a musical note produced upon the piano from that of the violin, or either of these from the tones of the human voice, or of a wind instrument such as the flute. Each kind of instrument produces a set of 248 Physiology of the Senses characteristic wave-forms, and the musician can tell by the effect produced what kind of instrument is sounding. The simplest form of vibration which gives rise to the sensation of a musical tone is that of a body vibrating in simple harmonic motion. Suppose a disturbance to be made in the perfectly smooth and level surface of a sheet of water. A concentric series of waves will spread out- wards from the point of disturbance .in ever-widening circles. But while the wave -forms move outwards, the particles which go to form the waves have only a vertical motion, up to the crest of the wave above, or down into the trough below, the ordinary water-level ; and after a series of gradually diminishing oscillations, they come to rest exactly in the position from which they started. If the waves were all of equal size the particles would move up and down in simple harmonic motion. Similarly, when a tuning-fork is vibrating so as to give forth a pure tone, its various parts move in approximately simple harmonic motion.1 If we attach a stylet to the limb of a tuning-fork, set the fork vibrating, and allow the stylet to write upon a sheet of paper drawn in the direction of the length of the fork, a curved line will be traced upon the paper similar to the curve from dQ to 8 in Fig. 125. The shape of the tracing will depend upon the rate at which the paper moves. If the paper moves slowly the waves will be short and steep ; if quickly, they will be elongated. Such a series of vibra- tions reaching the ear gives rise to a sensation which, lacking 1 A simple harmonic motion is thus mathematically defined by Thomson and Tait, Elements of Nat. Phil. Part I. p. 19 : " When a point Q moves uniformly in a circle, the perpendicular QP drawn from its position at any instant to a fixed diameter AAofthe circle, intersects the diameter in a point P, whose position changes by a simple harmonir motion." Sound and Hearing 249 brilliancy and variety, soon palls on the ear. The one continuous tone has a dull uniformity ; it is monotonous in every sense of the word. In the next place, suppose we have two tuning-forks vibrating at the same time but at different rates, and for the sake of simplicity let one of them vibrate twice as quickly as the other. We can now attend at will to the tone given forth by either fork, or to a new third sensation FIG. 125. — Pendular vibrational curves A and B. C, Vibrational curve obtained by superimposing B on A, so that the point e is on d§ ; D, vibrational curve obtained by superimposing B on A, with the point e on d\ of A. (Von Helmholtz.) produced by the combination of the two tones. If the waves of condensation begin at exactly the same instant, the combined effect may be graphically represented by the contiguous line in C, Fig. 125. When both forks produce condensation or rarefaction of the air at the drum-head at the same time, the effect will be that of the sum of the two. If one tend to produce condensation, while the other causes rarefaction, the combined effect will be equal to the 250 Physiology of the Senses difference of the two. Thus the height of the continuous curve C (Fig. 1 25) at the perpendicular c^ is equal to the sum of the height a^ d^ of wave A, and of the height of the crest at b^ in curve B. At d^ no effect is produced by B as the crest is changing to the trough. At a^ d^ A is still pro- ducing condensation, while B is producing rarefaction, the resultant effect being that at this phase the continuous line c falls below the dotted line between c^ c^ and so on. If the crests do not occur at the same moment, but at different times, as in D, the resultant form of wave will be different from that of C. Similarly in the case of the smooth sheet of water, if the surface be disturbed at two points the waves meeting and intersecting will have increased height or depth when crest meets crest or when trough meets trough, but if the crest of the one coincide with the trough of the other, the measure of the amplitude of the resultant wave will be the difference between the two. If the waves be of the same size and meet so that the crest of one exactly coincides with the trough of the other, they will counterbalance or neutralise each other, and the result will be a level surface for the water, or in the case of sonorous vibrations rest of the molecules and silence. And now let us suppose that we have an indefinite number of sets of vibrations, whose period or time of vibration is such that the primary or fundamental series is always a multiple of the smaller or more rapid sets, then the resultant curves, as graphically represented, may assume an infinite variety of forms, but these being repeated at regular intervals, the effect upon the ear will be that of a musical note. What complicated forms the wave may take can be readily imagined if we think of the effect produced on the surface of the sea by a gale of wind. The great rollers have their crests buffeted and broken by conflicting gusts, their surfaces roughened Sound and Hearing 251 by a thousand waves and ripplets. No two great waves seem exactly alike. Such a disturbance of the atmosphere affecting the ear would give rise simply to a noise, but let the great waves, irregular as they may be, succeed each other as exact copies one of the other, then we will have the musical tone, whose pitch or fundamental tone is that of the largest waves, but whose quality is determined by the combination of waves and wavelets into one. Resonators. — We can easily prove that the musical notes of most instruments are compounded of a fundamental and upper partial or overtones by using the resonators of von Helmholtz. These are hollow spheres of brass or glass with apertures to either side, as seen in Fig. 126, or tubes shaped somewhat like a bottle with- out a bottom. The air in these instru- ments vibrates at a given rate, or in other words, with a certain pitch deter- mined by the size of the resonator (the larger the resonator the lower the pitch), FIG. 126.— Resonator of and most loudly when a note of the von Helmholtz. same pitch is sounded in the vicinity of the resonator. When the smaller aperture is inserted into the external ear the special tone is heard to the exclu- sion of all others, the amplitude of the vibration being largely increased in the resonator. The principle by which this is brought about is the same as that which comes into play when any periodic motion is increased in amplitude by slight successive increments. For instance, suppose we wish to cause a person sitting on a swing to rise to a con- siderable height, or, in scientific terms, to cause the swing to move in vibrations of large amplitude. We first push the swing from the vertical, and thereby cause it to rise a slight distance above its lowest position. Under the influence of gravity the swing falls back to its position of 252 Physiology of the Senses rest, but acquiring momentum as it falls it passes the vertical line and rises on the other side until stopped by gravity, the friction of the rope, and the resistance offered by the air to the movement of the body through it. If, further, we ourselves interpose, we can readily prevent the rise and bring the swing to rest. But suppose we wait till the swing, having risen as high as possible, stops and begins to fall again and now give another slight push in the same direction as formerly. The new force added to the old, which has not yet entirely died away, causes the swing to rise a little higher than at first, and the return rise is also higher. Again, when it begins to fall we give a slight push, and so on, till at last the swing sweeps to and fro in wide oscillations and with great momentum. The periodic application of a slight force has given rise by summation of effect to a great force and extensive movement. So is it with the resonator. Vibrations of small amplitude in the external air set the molecules of air in the resonator into oscillation, and the successive impulses are given just at the moment when they will increase the amplitude of vibration. Thus atmospheric vibrations which, when diffused freely through the air, have insufficient energy to give rise to a sensation, will, acting upon the air in the resonator, set up a sympathetic resonance, which enables the ear to detect their presence even amid a multitude of louder sounds. But if the pitch of the external note is sharpened or flattened, the vibrations clash, and the resonator is silent. Analysis of Compound Tones by Resonators. — To satisfy ourselves that the sound produced by most musical instruments is compounded of many simple tones, we have simply to sound a note upon the instrument in question, and listen with a series of resonators. We will have, firstly, resonance for the fundamental tone, and then for a set of tones of higher pitch whose vibrational numbers are Sound and Hearing 253 multiples of that of the fundamental tone. We might have, for example, a set of overtones or partials or harmonics of the following relationship : — Fundamental Upper Partials or Harmonics. Note . . do1 do2 sol2 do3 mi3 sol3 sib3 do4 re4 mi4 Partial tones I 2 3 4 5 6 7 8 9 10 Number of\ vibrations / 33 66 99 132 165 198 231 264 297 330 Instead of applying a series of resonators to the ear, and so detecting the presence of various simple tones by hearing, we may analyse the compound note, and demon- strate optically the presence of the partial tones by means of an apparatus devised by Konig. This consists of a series of resonators mounted on a frame. The apertures of the resonators, which are usually inserted into the ear, are connected by elastic tubing with a set of small boxes. Coal-gas is led into the boxes, but prevented from passing to the resonators by closure of the entrance to the tubes with a thin india-rubber membrane. The gas passes from the boxes to a corresponding set of small burners, which give long pointed flames. When the air in one of the resonators is set in vibration, the membrane shutting off the resonator from the gas-box vibrates in sympathy, causing a variation in the pressure of the gas, and of the size of the flame. With all musical tones, however, the number of vibrations per second is so great that, from persistence of the retinal impression, we are unable with the naked eye to see the change in size of the flame. To obviate this diffi- culty, the rays of light from the flame are reflected to the eye from the surface of a cubical mirror rotating upon an upright axis. If the flame is burning steadily, the series of reflections of the light sent from the rotating mirror are blended into one smooth edged band of light ; but if the 254 Physiology of the Senses resonator is in action, the smooth band gives place to one with teeth on its upper border. Each tooth represents an increase of pressure from the resonator, each notch a diminu- tion. When a note containing the overtones to which these resonators respond is sounded, the flame picture in the mirror will declare their presence. The adaptation to organ FIG. 127. — Konig's apparatus for studying optically the vibration of air in organ pipes. pipes of the same kind of apparatus, viz. the gas-box, and the light of the flame reflected from a rotating mirror, is shown in Fig. 127, where we have the means of studying the vibration of air in organ pipes. By such an arrange- ment, for example, we may see that with two organ pipes sounding, the one an octave higher than the other, the Sound and Hearing 255 flame picture on the mirror for the upper note will have twice as many elevations as that of the lower. In the absence of von Helmholtz's resonators, a simple means of analysing a compound note, or at least of detect- ing its most important partial tones, is to cause the note to sound beside a piano. If we gently depress the key corre- sponding in pitch to that of the note sounded, so as to remove the damper, we will hear quite distinctly the sound of the piano-string vibrating in sympathetic resonance. Next depress the key of the octave above, and we will hear it sounding, but more faintly than the fundamental note. Again, if we press down the key of the fifth (sol) in the second octave, and so on with the various harmonic over- tones, we will hear the resonance, but always becoming weaker. It will, as a rule, be found that the sound obtained from any note other than those in the harmonic series is by no means so distinctly heard, although we may have in- harmonic upper tones due to a note being not purely musical in character, but accompanied in its production or propaga- tion by noises. For the notes sounded by almost all musical instruments, then, we may conclude that each note is compounded of a series of simple tones, each of which may be made to pro- duce its effect upon the ear as if the others were absent, and the total effect is due to a summation of the effects and a combination thereof to give a new sensation. We can imitate the notes of instruments having special overtones by combining pure partial tones, and in the organ some of the stops are so designed as to make sets of pipes sound together whose pitch is such as to give the effect of some other instrument, such as the flute, the clarinet, or even the human voice (the vox humana). As a result of a careful series of observations on the quality of different musical tones, the particulars of which 256 Physiology of the Senses are detailed in his book On the Sensations of Tone^1 von Helmholtz arrives at the following conclusions : — " I. Simple tones, like those of tuning-forks applied to resonance chambers, and wide stopped organ pipes, have a very soft pleasant sound, free from roughness, but wanting in power, and dull at low pitches. " 2. Musical tones, which are accompanied by a moder- ately loud series of the lower upper partial tones up to about the sixth partial, are more harmonious and musical. Compared with simple tones they are rich and splendid, while they are at the same time perfectly sweet and soft if the higher upper partials are absent. To these belong the musical tones produced by the pianoforte, open organ pipes, the softer piano tones of the human voice, and of the French horn. The last-named tones form the transition to musical tones with high upper partials ; while the tones of flutes, and of pipes on the flute stops of organs, with a low pressure of wind, approach to simple tones. " 3. If only the uneven partials are present (as in narrow stopped organ pipes, pianoforte strings struck in their middle points, and clarinets) the quality of tone is hollow, and, when a large number of such upper partials is present, nasal. When the prime tone predominates, the quality of tone is rich and full ; but when the prime tone is not sufficiently superior in strength to the upper partials, the quality of tone is poor or empty. Thus the quality of tone in the wider open organ pipes is fuller than that in the narrower ; strings struck with pianoforte hammers give tones of a fuller quality than when struck by a stick, or pulled by the finger ; the tones of reed pipes, with suitable resonance chambers, have a fuller quality than those without resonance chambers. " 4. When partial tones higher than the sixth or seventh 1 Von Helmholtz, Sensations of Tone, pp. 172, 173. Sound and Hearing 257 are very distinct, the quality of tone is cutting and rough. The reason for this lies in the dissonances which they form with one another. The degree of harshness may be very different. When their force is inconsiderable, the higher upper partials do not essentially detract from the musical applicability of the compound tones ; on the contrary, they are useful in giving character and expression to the music. The most important musical tones of this description are those of bowed instruments, and of most reed pipes, oboe (hautbois), bassoon (fagot), physharmonica (harmonium, concertina, accordion), and the human voice. The rough braying tones of brass instruments are extremely penetrat- ing, and hence are better adapted to give the impression of great power than similar tones of a softer quality. They are consequently little suitable for artistic music when used alone, but produce great effect in an orchestra." It has been stated that the quality of a tone is dependent upon the form of the wave which produces it. We have seen that the graphic representation of a complex tone reveals a series of very different forms of waves, according to the phase or period of combination of the partial tones. The question then arises : Does the ear appreciate these differences of phase in the combinations of partial tones ? For a given set of combined partial tones, do the different resultant wave-forms give rise to sensations of different quality ? To this question conflicting answers have been given. On the one hand, it is maintained by von Helm- holtz that " the quality of the musical portion of a compound tone depends solely on the number and relative strength of its partial simple tones, and in no respect on their differ- ences of phase." The difference of wave-forms C and D in Fig. 125, according to von Helmholtz, makes no difference in the sensation of the quality of the resultant complex tone. The ear has the power of resolving the complex vibrations 8 258 Physiology of the Senses into series of simple vibrations, and of hearing the pure tones corresponding to these sets of vibrations. As accord- ing to mathematical demonstration, however different the wave -forms for any given combinational tone may be, varying with phase of combination, these forms can only be resolved into one definite set of partial tones, the ear must always recognise the same set of partials, and we com- bine them again to give rise to a tone of the same quality. On the other hand, it is asserted that the different forms, representing as they do real differences in pressure on the drum-head of the ear, give rise to sensations of different quality. The curve D, for example, in Fig. 125, may be taken as representing short periods of increased pressure and long periods of diminished pressure upon the tym- panic membrane, while, by slightly altering the phase ot the component parts, we could give rise to alternate long periods of increased pressure and short periods of dimin- ished pressure. In the one case, the general condition is one of diminished pressure on the sensory apparatus with brief change to high pressure ; in the other, the sen- sory apparatus is subject in the main to higher pressure than usual, but with short periods of low pressure interven- ing. The pitch and intensity are, of course, unaffected, because the rate of vibration and amplitude of the waves are the same. The decision between the opposing opinions can be made only by personal trial, for, theoretically, we have no knowledge as to the way in which variations of pressure in the internal ear affect the sensory apparatus, nor, again, how changes in the end organ are transmuted into conscious sensation. As a matter of fact, the differ- ences of quality, if any do arise, are very slight, and only to be appreciated by a highly-trained ear, and with simple binary compounds. For the notes of ordinary musical instruments, or for combinations of numerous partials into Sound and Hearing 259 complex tones, it is practically impossible to detect differ- ences of phase, so that the statement holds good in the main that the quality depends, as von Helmholtz asserts, upon the number and relative strength of the partial tones. This holds for all perfect harmonies, at least those in which the vibrations are strictly periodic and resolvable into series of partial tones — the period of the fundamental tone being a multiple of those of the partial tones. Beats. — When two simple tones of exactly the same pitch are sounded together, if some arrangement be made by which the phase of vibration of each coincides, the result of their combination will be increased amplitude of vibration of the drum-head, and increased intensity of sound, but if the phase of one series of vibrations differ by a half wave-length from the other, the one will neutralise or interfere with the other, and there will be silence. Sup- pose, now, that we have two simple tones sounding together of the same intensity, and of nearly the same pitch — say, for example, that one is due to 200 the other to 201 vibra- tions per second — and suppose that the vibrations are in the same phase to begin with, it is evident that, since one falls behind the other to the extent of one wave-length in a second, it must fall one-half of a wave-length behind in half a second ; near the beginning and near the end of the second the vibrations are nearly in the same phase, and combine to intensity the effect ; but in the middle of the second, being in opposite phases, they tend to counteract each other, and there will be a diminution of intensity even to momentary silence. There will thus be an increase of volume followed by a diminution of volume of the sound every second, and we have an unevenness in the sound, or a succession of what have been called beats. The number of beats per second will depend upon, and be equal to, the difference of rate of vibration of the two partial tones. We. 260 Physiology of the Senses have seen that a difference of one vibration per second gives one beat per second. If the simple tones differ by two vibrations per second, there must be two beats per second ; for, since the one set falls two wave-lengths behind the other in a second, they must be one wave-length behind in half a second, and a half wave-length behind in a quarter of a second. There is increase of sound about the begin- ning of the first and third quarters, and diminution about the beginning of the second and fourth quarters, or, as we have said, two beats per second. Beats, then, can arise only when the vibrational number of one set is not a multiple of the other ; if the period of one is a multiple of the period of the other, there can be no beat. When there are not more than five or six beats per second, the ear can easily note the gradual rise and fall in intensity, and the effect is not unpleasant. When the beats come more quickly we lose the power of paying attention to the rise and fall of each beat, although we can still for a time recognise the beats as arising and differing from the continuous tones. The effect is that of a whirring harsh sound ; it is called dis- sonance. According to von Helmholtz, by gradually increas- ing the frequency of the beats, we may have as many as 1 32 per minute, and yet recognise the dissonant character of the sound and the presence of beats. Beyond this number the regular recurrence of the beats leads to a secondary fusion, and the starting of a new tone arising from the beats — a beat- tone. The ear fails to recognise a strictly musical character in beat-tones even when the beats are much more numerous than the vibrations required for an ordinary musical tone. This we may possibly explain by the fact that the develop- ment of beats is due not so much to a variation of pitch as of intensity. The higher tone continues to sound at exactly the same pitch as before, and there is merely a periodical variation in the amplitude of the vibrations which give rise Sound and Hearing 261 to it. We have, then, in the production of beats, a condi- tion analogous to the variations of pressure experienced in the sense of touch, in which, as stated (p. 58), we are able to discriminate the individual stimuli much longer than we can either with visual or ordinary auditory stimuli. There may be no fusion by the sense of touch of as many as 500 stimuli per second ; whereas, if the stimuli to the eye come faster than 10 per second, or by the ear 30 per second, there is a fusion in sensation. In the phenomena of beats, then, we seem to find a link between the sensation of touch and that of hearing, the tactile element (variation of ampli- tude) being superposed upon the auditory element (con- stancy of pitch). The unpleasantness of the sensation excited when the beats come at about 3 5 per second, when carefully investigated, is found to be similar in kind to that experienced when the senses of sight and touch are stimu- lated too rapidly for the bestowal of attention on each stimulus, and yet too slowly to give rise to central sensory fusion. A flickering light has a similar effect. The mind seeks, as it were, to maintain order in the reception of the messages of sense, to give to each sensation its due recogni- tion, and yet to subordinate it to general relationships and conscious sequence. But the stimuli come on the border- line between what may be grasped and what may not. Before the sensorium has had time to give full effect to one stimulus another has come upon it, and finds it partly ready but not quite, or, from the physical point of view, the sen- sory centre has not had time to recover completely from the disintegrating effect of one shock before it has to endure another. Something is being impressed upon the receptive centres which tends to force the mind from the path in which it seeks to move, and which is itself followed by another and another claimant for notice, till we become irritated at the disturbance and weary of the repeated dis- 262 Physiology of the Senses traction. All this, of course, takes place in a semi-uncon- scious way, since it is not, as a rule, the beat in the sound or the flicker in the light to which we wish to pay attention ; the pure musical sound with which the beat interferes, or the thing seen, now clearly, now dimly, in the changing light, is the object of mental effort. Without analysing the nature of the disturbing element, we feel that it is there, and to this must in the main be attributed the disagreeable effect produced. Yet while this holds true of long-prolonged tones roughened by fast-repeated beats, it must be remembered that in ordi- nary orchestral music we rarely hear notes entirely free from beats. While the various notes of a chord struck upon a piano may be of such pitch as not to generate beats, the overtones of these interacting on one another most prob- ably will. Certain chords, no doubt, are freer from such roughness, and it is no uncommon thing to heighten the effect of a pure harmonious note by causing it to be preceded by a discord. Contrast in sound, as in colour, heightens the effect on the sensorium. The eye fatigued by looking at a red colour will, when turned to a green surface, see it of intenser hue ; the ear has a keener appreciation of pure harmony when the harsh note has ceased to jar. Noise. — When auditory stimuli are non-periodic in char- acter the resultant sensation is that of a noise. A single variation of pressure upon the tympanum might be sufficient to set the mechanism of hearing in action, but the resultant sound could not be musical in character. It has been held by some that two impulses exactly alike, and the one quickly following the other, may give rise to a musical sensation, but the probability is that the musical effect is in this instance due to overtones, and to such a sound it is not possible to assign a definite pitch. The ear can easily distin- guish as separate noises the effect upon it of impulses coming Sound and Hearing 263 at the rate of less than 1 6 per second. When the noise is due to vibrations coming at the rate of more than about 1 6 per second, there is a certain amount of fusion in sensation, and the noise has for us a certain pitch. Where there is an initial shock, as in a thunder-peal, with echoing and re- echoing at somewhat prolonged intervals, we have a deep, rumbling sound ; if the vibrations succeed one another very quickly we have sounds or noises of high pitch, which we describe as crackling, whistling, rustling, shrieking, creak- ing, and so on. The wind sweeping through a forest sets up an infinite number of intermittent variations of aerial pressure as it sways branches and leaves to and fro, and a low rustling sound is heard ; but when it agitates tense structures, such as the cordage of a ship's rigging or the strings of an ^Eolian harp, the sound becomes more dis- tinctly musical, and especially if the wind blows with a fairly constant force. The harsh nature of the sound educed from a violin by an unskilled performer is due to inequalities of pressure upon the strings with the bow, while the master hand, by maintaining steady continuous pressure for longer or shorter intervals, and thus eliminating discordant over- tones, will draw forth pure melodious sounds. General Mode of Action of the Ear.— Having con- sidered the structure of the ear and the physical nature of sound, we have next to see how the one is adapted to the other, how the ear responds to auditory stimuli. Much may be learned from the study of pure physics as to the beauty of the mechanical adaptations, but this merely brings us to the threshold of sensation. The changes in the audi- tory nerves and nerve centres which accompany or give rise to the sensation of sound are almost entirely unknown. Even with regard to the mode of action of the internal ear there is still much uncertainty. 264 Physiology of the Senses The external ear, we have seen, acts mainly as a collector of sound waves, and the external meatus, closed internally by the drum-head, helps, like von Helmholtz's resonators, to increase the energy with which the membrane is agitated. The middle ear is so constructed as to diminish as little as possible the power of the aerial vibrations in their trans- mission to the sensory terminals. When vibrations pass directly from air to solids or liquids, much of their energy is lost. If a membrane intervenes between the air and a liquid, the energy is not lost to so great an extent. There is, therefore, mechanical advantage in the separation of the fluids of the internal ear from the air by the membranes closing the round and oval windows. But these membranes are small of size, tense in texture, and in apposition upon one side with fluid in an enclosed space. They have thus little amplitude of movement. This is compensated for by the drum-head. Being larger than the membrane of the oval window, and having air upon both sides, it vibrates freely, and being firmly attached to the tympanic ring and tense in the greater part of it, its vibrations are readily transmitted to the attached chain of bones, and by them, with little if any loss of power, to the foot of the stirrup-bone with its membranous attachment to the circumference of the oval window, and so to the perilymph. Nay, there may be an actual gain from the lever action of the chain of bones and the greater size of the drum-head (p. 213). The chain of bones, working freely in the middle ear, gives, as we have seen, a greater amplitude of movement than would be avail- able if the internal ear were simply buried deeply in the cranial bones. Still, the ligamentous connection of the bones with the membranes and the walls of the tympanum hinders over-movement, and enables them to act as dampers, pre- venting unnecessary oscillation of the drum-head. The tenseness of the membrane and, consequently, its power of Sound and Hearing 265 responding to sounds of different pitch and intensity are likewise regulated by the intrinsic muscles of the middle ear, and more especially by the tensor tympani muscle, while the entrance of air by the Eustachian tube maintains equality of atmospheric pressure upon the two sides of the drum-head. Vibrations then may reach the internal ear either through its osseous walls or through the membranes of the oval and round windows. In the vestibule and semicircular canals these vibrations are further transmitted to the membranous labyrinth through the perilymph, for the connection of this part of the auditory sac, with its surrounding walls, is by no means so close as in the case of the cochlear canal. Through the membranous sac the vibrations reach the endolymph, and so come to the terminations of the vestibular portion of the auditory nerve in the macula of the utricle and saccule, and in the cristce of the ampullae of the semicircular canals. The effect may be enhanced by the otoconia (p. 227) in the endolymph, and by the rods projecting from the auditory epithelial cells ; for, as has been pointed out, the hand thrust into water may be incapable of detecting the presence of sound waves passing through the water, but will easily do so if grasping a rod. This will be readily understood if we consider that the rod will act as a lever, and so increase the effect of the sound waves on the hand. That the auditory hairs do actually sway to and fro under the influence of sonorous vibrations may be taken as proved, for Hensen has seen with low microscopic powers the audi- tory hairs of My sis (the opossum shrimp) vibrating in response to the notes of a keyed horn. The auditory hair- cells are either the terminations of the auditory nerve fibres, or are in close apposition with them, and, on receipt of the vibrational stimulus, an impulse is given to the nerve ; but at this point we are arrested, for we do not know whether 266 Physiology of the Senses or not the nerve current corresponds in rate of intermission with the variation of pressure due to sound, whether vibra- tions are transmitted along the nerve, or whether we have to do with an entire change of physiological phenomena in the development of the nerve current. In the case of the cochlea, the vibrations may be trans- mitted by the perilymph, and through the membrane of Reissner and the cochlear endolymph, or through the basilar membrane to the endings of the cochlear branch of the auditory nerve in Corti's organ, or sonorous vibrations of the bones of the skull may, through the medium of the spiral osseous lamina and Bowman's spiral ligament, be directly transmitted to the basilar membrane and its super- jacent structures. From noting the mode of termination of the cochlear nerve in or round the hair-cells of Corti's organ, and from the analogy of the nerve-endings in hair-cells in the case of the other special senses, we cannot but infer that the hair- cells in the organ of Corti form the peripheral sensory ter- minals, while the rods of Corti and the supporting cells of Deiter, with their phalangeal connections, serve mainly to transmit to the hair-cells the vibrations set up in the basilar membrane. In all parts of the fluid of the internal ear changes of pressure due to movements of the chain of bones must be experienced, and as the fluid is incompressible, there must be an outward or inward movement of the membrane of the round window corresponding respectively to every inward or outward movement of the stapes. The question therefore arises : Do all parts of the internal ear, or at least, do all the terminations of the auditory nerve, respond alike to the sound; or does each nerve -ending have a special duty to perform, have a special response to a special element of the sound, be it pitch, intensity, or quality ? Sound and Hearing 267 The semicircular canals in relation to movements. — Con- sidered merely from an anatomical point of view, (we should expect a difference in function corresponding to the struc- tural differences between the maculce, cristce, and organ oj Corti, between the vestibular and cochlear divisions of the auditory nerve, and the different nerve centres to which they pass. It has even been suggested that the vestibular nerve and its terminals have nothing to do with the sense of hear- ing, but have to do with the sense of equilibrium or of the position of the head in space, while the appreciation of sound is relegated to the cochlea alone. In support of this view it has been pointed out that the semicircular canals, with their cristcz acustica, may be destroyed without impairment of the sense of hearing. At the same time, the animal be- gins to perform peculiar movements which vary according to the canal destroyed. If either of the canals in the vertical plane is injured, the animal rotates its head round a horizontal axis at right angles to the plane of the canal ; and, if the horizontal canal be injured, rotation takes place round a vertical axis. These rotary movements being similar to those produced by lesions of the cerebellum, and being apparently asso- ciated with a disturbance of the power of co-ordinating muscular movement— a power which depends largely upon the sense of equilibrium— it was held that the canals have to do with this sense, or, as suggested by Cyon in 1872, with sensation as to the position of the head in space. As Crum Brown has shown, the canals of the opposite sides of the head may be divided into three sets of two each in nearly identical planes, and so related as to be nearly at right angles to each other. When the head is moved in any direction, the fluid in the canals tends to move in the opposite direction, or at least to lag behind the moving walls of the canals, just as when we rotate a vessel contain- 268 Physiology of the Senses ing water the inertia of the water prevents its moving so quickly as the vessel at first, and of stopping so quickly when once set in motion. As the volume of fluid in the canals is constant, the fluid must, however, move with the head. It cannot lag behind, but there will be variation of pressure due to inertia. Thus, according to Crum Brown, " in each of the three pairs of canals (right and left hori- zontal, right superior and left posterior, right posterior and left superior) the two canals are so placed that when rota- tion takes place about the axis to which they are perpen- dicular, one of the two canals moves with its ampulla preceding the canal, so that the flow or tendency to flow (or pressure) is from ampulla to canal, while in the other the ampulla follows the canal, and the flow or tendency to flow (or pressure) is from canal to ampulla. If, then, we sup- pose that flow from ampulla to canal — or adopting Mach's view, increase of pressure in the ampulla — alone stimulates the hair-cells, while no effect is produced by flow in the opposite direction — or by diminution of pressure in the ampulla — we have in the six canals a mechanical system capable of giving us an accurate notion of the axis about which rotation of the head takes place and of the sense of rotation." * It has been further urged that the macula of the utricle and saccule have to do respectively with the sense of movement in a vertical or horizontal straight line, just as the cushions of the ampullae respond to rotation. On the other hand, it is alleged that even when the auditory nerve is destroyed and the body rotated, a sensa- tion of rotation comes on as usual. If this b£ so, the canals cannot be essential to the sense of position. Again, it is held that we cannot dissociate the vestibulas nerve from 1 A. Crum Brown, " Cyon's Researches on the Ear," Nature, 1878. See also M'Kendrick's Text-Book of Physiology, vol. ii. p. 694. Sound and Hearing 269 auditory sensation, since animals which can undoubtedly hear well may have a very rudimentary cochlea. On the whole, it seems probable that the vestibular nerve can respond to auditory stimuli. It may act under the stimulus of sound, and it may respond to differences of intensity of sound, but can it lead to the appreciation of differences in the pitch of sound ? To this question we must probably give a negative answer. No doubt, in the case of crustaceans, Hensen has found that auditory hairs of different lengths respond to certain notes better than to others, but no such difference of length in the auditory hairs of the macula or cristce can be seen in the human ear, nor any difference that could lead us to imagine that one cell should respond differently from another. The hairs on the hair-cells of Corti's organ are still shorter, so that we cannot conceive that they have any differentiating action as regard the appreciation of pitch. They seem to act rather, as suggested above, as minute levers by means of which the auditory cells are rendered sensitive to even the slightest movements in the fluid that bathes their free surfaces. Analytic Power of the Ear. — Has the ear, then, any mechanism which enables it to appreciate differences of pitch, or to analyse a compound tone into its constituent partial tones ? There is a fusion of all partial series of vibrations in the air of the external ear. The tympanic membrane vibrates as a whole, and responds to the com- pound summational wave, however complex its form may be — that is to say, however quickly it changes, and propor- tionally in extent to the variations of atmospheric pressure. With the drum-head moves the chain of bones, and with it again the perilymph and the endolymph. Yet, in the sen- sorium, we can appreciate either the quality of the complex tone, or we can attend to its constituent parts. Wherein 270 Physiology of the Senses comes the power of analysis ? Is it the case, as Ruther ford holds, that the hairs of all the auditory cells vibrate to every tone, just as the drum of the ear does, and that there is no analysis of complex vibrations in the coch- lea or elsewhere in the peripheral mechanism of the ear ; that the hair- cells transform sound vibrations into nerve vibrations, similar in frequency and amplitude to the sound vibrations ; that simple and complex vibrations of nerve molecules arrive in the sensory cells of the brain, and there produce not sound again, of course, but the sensation of sound, the nature of which depends, not upon the stimula- tion of different sensory cells, but on the frequency, ampli- tude, and form of the vibrations coming into the cells, probably through all the fibres of the auditory nerve ? J Upon this theory the whole internal ear vibrates in unison with the drum-head, and the auditory nerve in unison with both, just as the receiving plate of a telephone moves in unison with the transmitting plate. Analysis must then be a mental act dependent upon the powers of the central nerve cells, but how it is to be exercised we are not informed. Or does the power of analysis lie with the cochlea ? This is the theory which von Helmholtz first stated and explained with consummate skill. We have seen (p. 255) that when a compound tone is sounded before a piano with uplifted dampers, the strings of the piano which are in tune with the partial tones of the compound tone will vibrate. Similarly, von Helmholtz conceived that the cochlea has the power of analysing compound tones into simple pendular vibrations, and that different parts of the cochlea respond each to the particular partial to which it is attuned. At first, he sup- posed the rods of Corti's organ were the structures which, varying in size and shape, took up each its own tone, and, 1 Rutherford, "On the Sense of Hearing," The Lancet, January 1887, Sound and Hearing 271 by striking upon or otherwise exciting the hair-cells with which they were connected by means of the phalangae, caused sensory stimuli to be sent by the nerve fibres attached to the hair-cells to corresponding nerve cells in the sensorium. He did not, however, suppose that the nerve current re- sembled physically in any way the vibration which roused the auditory cell. The resulting sensation was simply due to the specific power of the cell in the brain, to give rise to a sensation of a sound of a certain pitch when stimulated by its proper tone. Various considerations, however, induced him to modify his theory. In the first place, the rods of Corti vary very little in form and size, as we pass from the base to the apex of the cochlea. Again, there are only about 3000 of them altogether, and yet we can distinguish differences of pitch in sounds varying in their number of vibrations from 30 to 40,000 per second. Further, we have good grounds to believe that birds can distinguish the pitch of tones, and yet the rods of Corti are entirely absent from their cochleas which have the hair-cells in contact with the basilar mem- brane, and are very rudimentary in other respects. For these and similar reasons, von Helmholtz supposed that the real analysers, in respect of pitch, are the fibrils in the outer part of the basilar membrane, and that the rods of Corti simply serve to pick up and transmit their vibrations to the hair-cells. This view is supported by the fact that the basilar membrane is stretched firmly in the direction of these fibrils, but is loose in the direction of the canal. The fibres are easily separated from one another, but are not readily torn across. The membrane will not vibrate, as a whole, like one in which the tension is alike in all directions, but it is made up of strings or fibres, each of which may vibrate independently of the other. There are about 24,000 of these fibrils in the basilar 272 Physiology of the Senses membrane — a number much larger than that of the rods of Corti, although less than the number of sounds between which we can make a distinction of pitch. Von Helmholtz supposed, then, that these fibrils, varying in length and possibly in tension, may respond in sympathetic vibration each to its proper tone, and that these vibrations are trans- mitted to the hair-cells by their supporting structures. If a tone falls upon the ear which does not correspond exactly in vibrational frequency with that of any of the fibrils, von Helmholtz suggested that two or more adjacent fibrils might respond in various degrees, that being strongest which approximated most nearly to the stimulus, the others more feebly. By a mental combination and comparison of the different stimuli the true pitch of the note would be arrived at Thus each fibril has, according to him, one proper tone to which it answers strongly, while to all others it is less responsive. Similarly, in the case of the stimulation of the auditory hairs of My sis, it was found that different hairs responded strongly to different tones. One, for example, vibrated strongly to d§ and d'§> more weakly to g, and very weakly to G. Another hair answered strongly to a{ and adjacent tones, more weakly to d§ and Ajf. For some tones, then, the cerebral cells are directly tuned, but not for others ; for all others there must be a comparison of several tones and appreciation of pitch through the means of an average. As von Helmholtz does not suppose that the nerve current in any way corresponds in number of vibrations to that of the exciting cause, each nerve cell depends on its own inherent power of response in giving rise to a sensation of a special pitch. But, further, it has been computed that there are only about 15,000 hair-cells, and if it be the case that each of these is connected with one nerve fibre and its special brain cell, and that each hair-cell corresponds only to one tone, the number of special tones to be directly recognised Sound and Hearing 273 in the brain is considerably less than the number of fibrils of the basilar membrane would lead us to expect. On the other hand, if the cell may respond to more than one tone, and give rise to sensations of different tones in the sen- sorium, ^ve must have some difference in the nerve currents transmitted at different times from periphery to centre by the same nerve, and this would probably correspond to different rates of vibration of the basilar fibrils. Now, it is just possible that there may be a greater power of response in the basilar membrane to sounds of varying pitch than von Helmholtz supposes. If at any particular moment there is no fibril attuned to the pitch of the incoming sound, it may be that the tension of part of the membrane may be varied to suit the exigencies of the case. We have seen that Bowman's ligament, by which the basilar membrane is attached to the outer wall, contains spindle cells which may be regarded as muscular, and by the con- traction of which the pull upon the fibrils may be varied, and their tension increased or diminished. A similar result might follow a change in the amount of blood circulating in the spiral ligament, giving more or less turgidity to this structure. Thus if each fibril of the basilar membrane in its normal condition of length and tension is tuned approxi- mately to a special tone, and if by variation of its length or tension it may be rendered responsive to tones of slightly higher or lower pitch, as we may tune a violin by tightening or slackening the strings, we have in the ear a complete analysing mechanism for the pitch of all musical sounds. Such an hypothesis renders it possible likewise that we may have a complete series of tones from the lowest to the highest, melting one into the other by imperceptible change — an ear, in fact, that can appreciate the pitch of any possible tone between the lowest and the highest limits, a capacity which experience shows to be possible in the human ear, T 274 Physiology of the Senses and that directly for all tones, and not indirectly for some, as von Helmholtz holds. If, further, it is the case, as Rutherford suggests, that the sensation varies in the central cell according to the rate at which the peripheral end of the nerve fibre or the, hair-cell is stimulated, we arrive at a view which is free from objec- tions that may be urged to the theories both of Rutherford and von Helmholtz. Rutherford's theory is unsatisfactory in so far as it entirely disregards the elaborate structure and wonderful complexity of the cochlea, deprives the ear of any analysing power, and relegates that function to the brain, among whose cells we can find nothing in any way suitable, from a morphological point of view, to lead to a perception of variation of pitch. The physical basis for analysis must be either in the ear or the brain ; but if all parts of the ear, and all the fibres of the auditory nerve, and all the auditory nerve cells, respond together and vibrate alike, we have no such basis. To have the power of selecting one or other partial tone, and of devoting attention to it alone while others are still affecting the sensory mechanism, it seems to us that there must be several structures in vibration or molecular change at different rates. If the auditory centre is in vibra- tion or molecular action as a whole, and similarly in all its parts, it is impossible to understand how a mere effort of will can enable us to note constituent parts of a complex tone. We can pay attention to one or other partial tone in a complex sound, just as we can fix our regard upon one part of the field of vision to the exclusion of all the rest, but how can this be done if all parts of the auditory centre are affected alike ? To each part of the retina there is a cor- responding part in the cortex of the brain ; there is probably a similar relationship between different parts of the cochlea and the auditory centre. On the other hand, the main objections to von Helm- Sound and Hearing 275 holtz's theory are the limited number of structures compared with tRe known capacity of the ear and the supposition that each brain cell is concerned only with the perception of one tone in different degrees of power. All are agreed that the cerebral centres can appreciate variations in strength of stimulus. In all the special senses the strength of the sensation varies with the strength of the stimulus. Now, this does not necessarily imply in regard to the auditory nerve that the actual vibration of the endolymph is trans- mitted as a vibration that might be seen passing along the auditory nerve as we might see a wave of vibration passing along a tensely-stretched rope when it is struck, but it does imply a greater molecular movement in one case than in another, and a greater or less effect upon the proto- plasm of the receptive nerve centre. There may be no real to-and-fro vibration of the nerve corresponding to that of the internal ear, but there must be a variation in the nerve current in respect of amount of movement. If the nerve cell can respond to variations in intensity, there is no greater difficulty in supposing that a cell whose function is to 'give rise to a sensation of pitch may give slightly different sensa- tions corresponding to slight variations in the rate of stimu- lation.1 If it be urged that this again relegates distinction of pitch to the brain, and that we might as well suppose each auditory cell to have the power of discriminating between all degrees of pitch, we would answer that the multiplication of centres, each having slightly different receptive powers, affords an anatomical basis for the simul- taneous reception of many stimuli differing from one another 1 See also the remarks on the modified theory of colour vision recently propounded by von Helmholtz (p. 169). This distinctly favours the view that terminal organs, such as the rods and cones of the eye (and why not the delicate mechanism of the internal ear ?), may respond to different rates of vibration. 276 Physiology of the Senses only it may be in the matter of pitch, while by allowing that each little centre may give slightly different pitch- sensation with variation in the rate of stimulus we avoid the difficulty into which von Helmholtz's theory plunges us. But, it may be asked, can a nerve fibre respond in this way to different numbers of stimuli per second ? There is not the least doubt that it can. The number of stimuli sent along a nerve to a muscle may be largely varied with varying effect on the muscle in the way of contraction. In the case of insects, for example, the wings may vibrate as often as 352 times per second (Rutherford), and each movement must be due to at least one separate nerve impulse. A nerve removed from the body may be inserted in a tele- phonic circuit, and it will conduct the electric current and transmit the delicate variations of electrical intensity neces- sary for telephonic communication. We do not assert that the ordinary nerve current is electrical in character, but if the nerve can transmit variations so delicate as those of the telephone must be, they may as readily be deemed capable of responding in rate to their normal auditory stimuli. Moreover, it must be borne in mind that the sensation of pitch is in no way comparable qualitatively with the phy- sical changes which give rise to it. We have no sensation of each individual variation in the stimulus. The sensorium fuses the impulses so as to. give rise to a continuous tone. And again, we do not, as a rule, note the partial tones separately and respectively : indeed, until the time of Tar- tini they were not known to exist, and until the time of von Helmholtz were deemed of small importance. Their com- bination and appreciation, as a sound of determinate quality, is a purely mental act, combined, that is to say, by a mechanism higher than and different from the initial recep- tive auditory centres. It is only when, by conscious effort and using special aids, such as resonators, we pay attention Sound and Hearing 277 to the sensory effect that we note the constituent parts. There must be higher mental centres in which fusion occurs, or a unity of mind in which a synthesis of the partial sen- sations is brought about. The Psychical Elements in Auditory Sensations.— When the auditory centres have been stimulated and the sen- sation of sound receives due attention, certain mental effects are produced which are superadded to the simple sensation of sound. We judge, for example, that the sound has been produced outside or inside of the body, that it comes in a certain direction and from a certain distance, or we may recognise that it is purely of a subjective character, and exists only in imagination. In arriving at a decision upon such points as these we are aided by the other senses and by knowledge previously acquired. Thus, when we see a man at a distance from us lifting a gun to his shoulder and a puff of smoke issuing from the muzzle, we know from experience that we will shortly hear the sound of the detona- tion. We infer from the character of the sound, its loudness, and the time that elapses before the report is heard, that it comes from the gun and from no other source. Externality of Sound. — The power which the mind possesses of determining whether a sound originates out- side or inside of the body seems to be in large measure dependent upon whether the sonorous vibrations are com- municated to the ear through the auditory meatus, the drum-head, and the chain of bones, or directly through the bones of the head. We mentally project the source of the sound outwards when the vibrations act mainly through the meatus on the tympanum, but if the sounding body is touching the head we may have the impression as if the sound came from within the head. Weber has pointed out that if the meatus is filled with water the idea of externality is 278 Physiology of the Senses destroyed, and that the sound seems to originate in the head Even when the air in the meatus is vibrating freely in re- sponse to sonorous undulations, if the body emitting the sound touches the head, the idea of externality may dis- appear. Suppose two bodies giving out exactly similar sounds, as when two telephones, connected in one circuit, are held to the two ears and made to respond to one and the same sound. If the telephone to the right side be tightly applied, while the one to the left be held at some little dis- tance from the ear, the sound will seem to originate in the right side of the head. If the one to the left is now pressed closely and that to the right withdrawn a little, the sound is heard in the left side of the head, but if both instruments are held tightly to the ears, the sound seems to originate inside of the head and towards the middle line, so that it will be described by one observer as seeming to be in the mouth, by another at the top of the head, and by a third at the nape of the neck. Lastly, by slight variations in the pressure on the head we can apparently make the sound move from side to side at pleasure. The sound of our own voice is heard as originating within the head, and certain disorders may give rise to sensations of sounds re- ferred to the ears. Thus when the intracranial circulation has been disturbed, we may have a ringing in the ears, or may hear the throbbing of the pulse. An accumu- lation of cerumen or wax in the external meatus may give rise to unpleasant sounds by interfering with the vibration of the drum-head. Drugs, such as quinine or salicin, may cause hissing or whistling sounds, or even a sensation of deafness, by interfering with the nutrition of the auditory centres, and the insane often think they hear voices and sounds on account of disordered and abnormal stimuli in the diseased brain. So strong, indeed, is the power of imagination in the hallucinations of the insane that nothing Sound and Hearing 279 will persuade them that the voices are not actually coming from an external source, and it is to be remembered that the sensations are at least real to them, latent impressions being developed or obscure memories recalled by cerebral irritation. Nay more, we may ourselves under certain cir- cumstances by an effort of the mind give rise to auditory hallucinations. Much pleasure may often be derived from the following experiment. If when in bed, lying perfectly quiet, and with no sounds breaking the stillness of the night, we think the music of a song, fixing our attention upon the music but not humming it, we may sometimes seem to hear it being sung an octave higher by a voice ex- ternal to ourselves — a female voice apparently, from its delicacy, tenuity, and high pitch — and, strange to say, not exactly synchronous with but very slightly behind our own imaginary singing. When the hallucination is thoroughly established and we resign ourselves completely to it, the two voices may seem to go on without effort on our part, and we ourselves to be merely passive listeners. The least movement, however, or wandering of the thoughts to another subject, immediately dispels the illusion. In per- forming this experiment, it is most probable when the mind has all its faculties concentrated upon the endeavour to hear the faint sound that, in thinking the music, we actually give rise to slight variations in the tension of the auditory structures, and possibly stimulate the auditory centre through the auditory nerve, but to so small an extent as to be hardly perceptible to the senses, or it may be that with the concentration of the mind upon the expected sound the nutrition of the auditory centre is involved. It might even be that the auditory centre is stimulated from the parts which subserve volition, but this is mere conjecture, for which no experimental data can be adduced beyond the well-established fact that lower centres may be inhibited or 280 Physiology of the Senses excited by influences coming from higher cerebral centres. As a monarch may summon his ministers and invoke their aid or dismiss them from his presence, so the conscious mind may call upon the senses for their testimony, or may bid them be silent, and the obsequious senses do some- times seem to give that answer which their master desires, although they have no true warrant for so doing. Direction of Sound. — We have seen (p. 200) that the determination of the direction in which a sound has come is largely due to the greater intensity of the sound in one ear than in the other owing to the sound waves striking more fully and directly upon one ear than the other. If, however, the source of sound is in a plane passing forward through the middle of the body it is impossible by means of this alone to say whether the sound comes from behind or in front. Judgment as to direction is made more accurate by moving the head so that the sound falls more intensely now on one side now on the other. If the apex of a hollow cone or the ear-piece of an ear-trumpet be inserted into the meatus and the instrument be moved for- wards and backwards, the apparent direction of the sound may be largely modified, and we have a similar change if the auricle be flattened out backwards against the side of the head or brought forward with the hand. In many cases, we judge the sound to come in a certain direction from knowing where it probably originates, as when we hear a bell rung in a steeple with whose position relatively to ourselves we are acquainted. It is easier to judge the direction of noises than of musical sounds, and that mainly because there is a slight difference in the quality of the sounds coming to the two ears, and noises having generally more partial tones than musical sounds, the differ- ence is more easily noted and the, judgment as to direction assisted. Sound and Hearing 281 Distance of the Source of Sound. — The ear has no direct power of estimating the distance from which a sound comes, since it only becomes cognisant of the sound when it reaches the ear. We can only form a rough estimate from knowing by previous experience that a given sound will presumably have a certain intensity when produced at a certain distance from us, and that, other things being equal, it will diminish to a certain extent the farther it is from the ear. Experimentally, it has been proved that when sound is transmitted through a fairly homo- geneous medium, as through air or water, the intensity of the sound varies inversely as the square of the distance. For twice the distance, the intensity will be one-fourth ; for three times the distance, one-ninth, and so on. But if we modify the conditions for the transmission of sound, our power of judgment soon fails us. If, for example, when sitting at a table we scratch it gently with the finger-nail, the arm being outstretched, we hear a sound of faint inten- sity, the distance of which we can estimate fairly well ; but if the ear be applied to the table, the sound seems to be made at the ear, its intensity not having been materially diminished by transmission through the wood. Similarly, if the sound is transmitted through tubes, the law of diminu- tion of intensity, according to the square of the distance, does not apply, and we hear people speaking through a long tube, as from top to basement of a house, as if they were close beside us. By gradually diminishing the intensity of a sound, it may be made to seem to come from a consider- able distance when really being produced close at hand. Thus, when the operatic chorus leaves the stage, and dis- appears from view behind the scenes, by singing more and more softly, the performers can convey the impression that they have retired to a great distance. So the art of the ventriloquist lies in his power of speaking with almost no 282 Physiology of the Senses facial movement, of changing rapidly the strength of his voice so as to give the impression of varying distance, and of conveying by gestures that the sound seems to come from a certain spot, whence he seems to hear it coming, just as we do ourselves. A slight variation in the quality of a sound likewise takes place as it recedes from us, certain partial tones becoming inaudible sooner than others ; this too may help our judgment as to distance. Memory of Sound. — It is sometimes difficult for us to judge by the power of hearing when a sound has ceased to stimulate the ear. When, for example, a bell has been ringing for some time and then stops, the sound gradually dies away, and it is almost impossible for us to tell the exact moment when it has ceased. It may seem to have died away entirely, and we cease to strain the ear to catch its faint tones, but if we listen again we seem to hear it faintly. This may be due to different causes. It may be that the ear has become fatigued for the special sound, and that the momentary withdrawal of the attention has rested the ear, so that it can respond to tones previously inaudible. On the other hand, it may be due to a vivid form of memory. We cannot doubt that there is some physical change in the auditory centre when the sensation of sound is excited, and when the centre has once acted in a particular way it does so more easily when similar circum- stances again arise, or even as the result of a mental effort. Sometimes it may require repeated attempts before we are able to recollect a sound, as, when after hearing a new song, we fail for a day or so to remember the music of it, but gradually note by note, and line by line, it returns, often without conscious effort, until we are able to piece it all together again, more or less correctly, according to acute- ness of ear and receptivity for musical impressions. Mental Receptivity for Sound. — This is a power which Sound and Hearing 283 varies much with the state of the mind and the nature of our environment. As a rule, we pay no attention to, and do not consciously hear, such customary sounds as the tick- ing of a clock, the noise of street traffic, and the like, although they must be constantly acting upon the ear. They, indeed, constitute for us our basis of silence, so to speak, for if the clock should stop, or if we pass to the solitude of the country, we seem to hear the silence which ensues. Again, just as some people are colour blind, so others may be deaf to the pitch of sounds. Some ears are adapted only for sounds of comparatively low pitch, others for high pitch ; they are deaf to all others. If we take the lowest limit for pitch at 1 6 vibrations per second, and the highest at about 40,000, we have in all a range of about 1 1 octaves. The ear has thus a much wider range for pitch than the eye for colour, for it will be remembered that the lowest red rays of the spectrum have a vibrational frequency of 435 millions of millions per second, while those of the ultra violet are about 764 millions of millions — that is to say, less than twice the number at the lower end of the spectrum, or less than one complete octave. But the power of distinguishing tones of varying pitch is, with some, so slight that they are quite unable to distin- guish one tune from another, and others who can recognise the difference are unable to sing more than one or two notes of different pitch. Binaural Audition. — Some persons have been found who seemed to have the two ears differently tuned, so that the same sound seemed to be of higher pitch to one ear than to the other. Under normal conditions, although from the position and shape of the ears the sound waves which fall upon the drum -head cannot be exactly the same in form nor in time of excitation, yet the resultant sensations in the auditory centre are mentally united, and 284 Physiology of the Senses we hear one sound, not two. This is mainly to be accounted for by the fact that the sensation lasts for a short time after cessation of the stimulus, and the two sounds are so slightly separate in time as to blend readily with one another. Inasmuch as the two ears enable us to a cei- tain extent to judge the distance of the sounding body, binaural audition is, in a way, comparable to binocular vision, which assists in the perception of solidity or distance in space. THE PHYSIOLOGICAL CONDITIONS OF SENSATION IN the preceding sections we have given, in the first place, a general view of the mode of action of the nervous system, and then we have described each of the five senses in detail. We have seen that external agents, such as light or sound, act on special terminal organs, and that from these, nervous impulses are carried by the nerves of sense to the central nervous organs. In these central nervous organs molecular changes occur, which are related in some way to conscious states or sensations, and we then refer these sensations to the outer world, and to the agent which we believe to be their primary exciting cause. Further, we know that these sensations may give rise either to voluntary or involuntary movements, and that they may influence many organs of the body, causing, for example, the voluntary movement, the involuntary start, the blush of modesty, or the pallor of fear, the more rapid action of the heart, or the quickening or slowing of respiration. The functions of the central nervous organs and of the organs of sense are so closely related as to make it no easy matter to form a conception of the system working as a whole. The progress of discovery naturally tends to differentiation, and to attaching undue importance to one organ as compared with others, so that we are in danger 286 Physiology of the Senses of losing sight of the solidarity of the whole nervous system. During the profound unconsciousness of coma, or of deep sleep, the mind is at rest. There are no thoughts and no interpretation of messages from the sense organs. The higher centres of the brain are inactive, but lower centres, such as those governing the circulatory and respiratory mechanisms, may still be active, the heart continues to beat, and an onlooker sees the movements of respiratipn. During the waking and conscious state, however, the higher centres are active. They are not only the seat of molecular phenomena related to the conscious state, giving rise to the revivications of memory, the play of ideas, the rise of desires and impulses, and efforts of volition, but they now are momentarily receiving messages from the various sense organs. These messages affect the higher centres them- selves, and, through them, lower centres and the body generally. Probably every nervous action, however deli- cate and evanescent, affects more or less the entire system, and thus, in addition to the impulses coming from the various organs of sense, there may be an undercurrent streaming into and out of the nerve-centres. This under- current may never give rise to distinctly conscious states, but, along with numerous interactions in the centres them- selves, it contributes to, and partly accounts for, the appa- rent continuity of conscious experience. No one doubts that consciousness has a material sub- stratum, but the problem of the relation between the mental state and the molecular movements in nervous matter is as far from solution as in the days when little was known of the physiology of the nervous system. Con- sciousness has been driven step by step upwards until it now takes refuge in a few thousand nerve-cells in a portion of the gray matter in the cortex of the brain, or it may be Physiological Conditions of Sensation 287 in the dense network of fine fibrils that abounds in gray matter. The ancients believed that the body participated in the feelings of the mind, and that the heart, liver, and reins (kidneys) were connected with the emotions, a view quite consistent with the familiar experience that these organs are often influenced by such mental states. As science advanced, consciousness was relegated to the brain, first to the medulla, and lastly to the cortex. But sup- posing we were able to understand all the phenomena — chemical, physical, physiological — of this intricate gan- glionic mechanism, we would be no nearer a solution of the problem of the connection between the objective and sub- jective aspects of the phenomena. It is no solution to resolve a statement of the phenomena into mental terms or expressions, and to be content with an exclusively idealistic theory of cognition. Nor is it more satisfactory to translate all the phenomena of mind into terms describ- ing physical conditions, as is done by those who support a purely materialistic hypothesis. A philosophy that recog- nises both sets of phenomena, mutually adjusted and ever interacting, recognises the facts of the case, and does not delude the mind by offering a solution which is in reality no solution at all. The difficulty is somewhat lessened if we assume that behind all physical and mental phenomena there is a metaphysical essence, conscious or unconscious, and that the phenomena we term physical and mental are only different sides of the same thing. Such an essence can never be known to science, and the discussion of the possibility of its existence and of its properties belongs to the province of philosophy.1 Apart from the ultimate question, however, there is the important one whether physiologists are right in relegating consciousness entirely to the gray matter of the brain. The 1 Von Hartmann, Philosophy of the Unconscious, especially vol. iti, 288 Physiology of the Senses facts of comparative physiology are against a view so exclu- sive, because we cannot deny consciousness to many animals having rudimentary nervous systems, or none at all. As already said, research in anatomy and physiology, and the observation of disease, have obliged physiologists to adopt the view that the brain is the seat of sensation, or, in other words, of consciousness. This is no doubt true in the sense that it receives all those nervous impulses that result in con- sciousness, but parts acted on by external physical agents (like the retina) and the parts transmitting the nervous impulse (like the optic nerve) are, in a sense, as much con- cerned in the production of conscious states as the brain itself. This view of the matter was urged by Cleland in 1870,! and is consistent with the facts of nervous physio- logy. It presents fewer difficulties than the one generally held which drives consciousness into the recesses of the nerve-cells in the cortex of the cerebral hemispheres. It keeps clear of the prevailing error in the philosophy of modern physiology, that of regarding the body, and even the nervous system, as a vast collection of almost inde- pendent organs, losing sight of community of function and interdependence of parts. At the same time it must be admitted that it approaches no nearer a final solution of the problem of the origin of consciousness ; it only states the conditions of consciousness with greater precision. Let us now approach the question from another point of view. The simplest structural nervous unit is a Cell, which we may call A, with a fibre passing to it from a specialised cell, B, on the surface of the body, and another fibre passing from it to a contractile cell, C. A stimulus applied to B causes molecular changes in it, which result in the transmission of an impulse to A, in which molecular changes again occur, resulting in the transmission of an 1 Cleland on Evolution, Expression, and Sensation, 1870. Physiological Conditions of Sensation 289 impulse to C. This is the simplest form of a so-called reflex mechanism. Suppose the same kind and degree of stimulus be applied to A many thousand times in succes- sion, and repeated not only in an individual, but in a line of individuals genealogically connected as parent and offspring, we can imagine that its molecular structure will become so modified that it will gradually become more and more responsive to stimuli of this kind, the simple mechanism having become attuned to the movements of the outer world. Here, then, we have a molecular condition associated with the dawn of consciousness, and the attuned condition of the structure may be regarded as the beginning of memory. No doubt it is impossible here, just as in dealing with a complex brain, to form any conception of the genesis of consciousness. It evidently cannot be the result, in any physical sense, of the molecular changes in the cell, because even although we were cognisant of all the molecular changes we could not detect a conscious state. So far as an out- sider is concerned, the conscious state of the cell can only be recognised by some outward manifestation in the form of movement, and it is conceivable that the cell might be conscious, and yet not make any movement. Suppose a, 6, c, d, e, etc., to represent links in the chain of physical phe- nomena between the irritation of the cell B and the move- ment of 0, and that consciousness is an attribute of A, which we may denominate x, it will be impossible to find a place for x in the chain, in the same sense as the movement of 0 is the last link of the chain. It cannot come in be- tween a and £, as a is the physical antecedent of £, nor, for a similar reason, between b and c, nor between c and d, d and *, etc. The condition x is therefore outside the physical chain ; and yet it is related to it so intimately as to lead to the illusion that x forms one of the links. This appears to prove that consciousness, x, is outside 290 Physiology of the Senses any chain of related physical phenomena conceivable in the simplest nervous mechanism. Nor do we get any farther towards clearing up the mystery if we suppose, as some have done, that even dead matter has in some way associated with it units of con- sciousness,1 because it is equally impossible in this case to understand the nexus between the material particles and consciousness. The condition of the conscious state may therefore be represented by two parallel curves infinitely close together, the one representing the chain of physical phenomena, linked together as cause and effect, and the other the chain of conscious states. Any variation in the one coincides with a variation in the other, but no explana tion can be given as to how the one influences the other. To assert that one is the cause of the other is simply to beg the question. If we say that the chain of physical phenomena is the cause of the conscious states, in the same sense as the physical phenomena in a cell of the liver is the cause of the secretion of bile, we introduce into the chain an immaterial something, and break the physical continuity of the various links ; and, on the other hand, if we try to escape the difficulty by translating the physical links themselves into states of consciousness, and deny any knowledge of the physical substratum, we are deceived by words and reach no solution. Again, to regard consciousness as a mode of energy is unsatisfactory. Energy, in the physical sense, is nothing more than the power any material system has of doing work, owing to the relative position of it's component parts. If the relative position of these parts be altered, the distri- bution of energy in the system will also be altered. It follows from this that energy may be manifested by various kinds of movements — heat, light, gravitation, etc. — and 1 W. K. Clifford, Lectures and Essays, vol. ii. p. 31. Physiological Conditions of Sensation 291 one form of energy may be resolved into another. But when motion produces heat, there is a quantitative con- version of energy from motion to heat, which is, in turn, another mode of motion. If we now assume molecular changes to be the cause of consciousness, these molecular changes also produce heat, molecular movements associated with chemical action, and perhaps movements on a larger scale ; but the sum of these resultant forms of energy is equal to the energy at first existent in the physical system, which we assume to be also the seat of consciousness. Con- sequently consciousness does not come into the dynamical chain. It cannot be measured ; it cannot be derived from the physical energies, nor can it be resolved into them. It is outside the chain. Movements of matter, therefore, cannot be resolved into consciousness, or, in other words, conscious- ness is not a form of energy. We are thus face to face with an insoluble problem, even when we discuss it in its simplest form, and it becomes infinitely more complicated when we consider the manifold phases of consciousness connected with the mechanism of the brain. If, however, we begin with the structural unit of a simple reflex mechanism, along with its associated conscious state, we find that the complex functions of the fully - developed brain are aggregations of the simple mechanism we have considered, and that what we term consciousness is a condition which is the sum of the conscious states of the individual nerve cells, or aggrega- tions of nervous matter, constituting the brain. We can form no conception of the nature of the consciousness of a nerve cell any more than we can of the consciousness of a sea -anemone or of a worm ; but we must assume the existence of consciousness in a nerve cell, otherwise it is impossible to understand how consciousness is associated with an aggregation of such cells in a brain. To deny 2 92 Physiology of the Senses consciousness to such a cell would be equivalent to deny- ing consciousness to the brain, which would be absurd. Whilst, therefore, we give up the explanation of the genesis of consciousness as an insoluble problem, it is possible to gain some insight into the general mode of action of brain as the recipient of sensory impressions. Suppose, for example, we irritate the skin of the sole of the foot, an impulse is carried by nerves to cells in the posterior horns of gray matter in the spinal cord (see Fig. 7, p. 1 6), in which molecular processes are excited. From these, impulses are carried by fibres in the cord to cells in the anterior horn ; in which, again, molecular processes occur, resulting in the transmission of nervous impulses along motor nerves to the muscles of the limb, and the limb will be drawn away by a sudden contraction of the muscles. This is a reflex movement, not in obedience to a volitional impulse, not associated with consciousness in the usual sense of the term (as implying activity of the brain), but, from the arguments already led, we may assume that these molecular changes in the cells of the cord are associated with a lower mode of consciousness, such as presumably exists in animals having a nervous system of this simple type. But the cells in the gray matter of the cord are connected with cells in the masses of gray matter in the upper centres, and, in particular, we have reason to believe that each unit area of sensitive surface of the body has a corresponding unit area in the cerebral cortex, that is to say, from each unit area (the size of which varies much in the different sense organs, from a minute area of retina to a much larger area of skin surface) nerve filaments pass which carry impressions to a corresponding unit area in the cortex (see remarks on the tactile field, p. 60, and on the visual field, p. 30 and p. 176). This does not mean that individual nerve fibres necessarily pass from unit area of sensory Physiological Conditions of Sensation 293 surface to unit area of cortex, but that impressions are so related. If so, the irritation of the skin of the foot, in the experiment we are considering, may cause impressions to pass, not merely to the cord, but also to the higher centres in the brain, and the result may be a feeling of pain. This may be also explained by supposing that the reflex centre in the cord is intimately connected by fibres with the conscious centres in the cortex, a supposition strongly supported by the increasing mass of evidence as to the paths of transmission between the cord and the brain. The sensation of pain must be associated with molecular changes in the cells of the cortex, and, as a rule, these changes cause, by a kind of irradiation, the transmission of impulses outwards to other nerve centres, which in turn call forth various more or less complicated movements. Thus, for example, they may be carried to the cells in the gray matter of the medulla, which is the origin of the nerves governing the movements involved in crying, in the ex- pression of pain by the muscles of the face, or they may reach the cells in the gray matter of the cord, calling forth the movements of the limb requisite for drawing the limb away from the irritation, or for defending it from further attack. Again, the irritation may call forth involuntary exclamations, in the form of words, expressive of pain, and in this case the centre for articulate speech has been in- volved. Impressions may also be carried from the sensory centre in the cortex to the parts of the brain concerned in volition, and the reflex and involuntary movements we have considered will be added to, or supplanted by, direct voluntary movements. Even voluntary movements, how- ever, are essentially reflex in character, inasmuch as they are called forth by stimulations which have been applied to a sensory surface either immediately before the voluntary act, or which have been applied, it may be, long before. 294 Physiology of the Senses In the latter case, the effects of the stimulation still remain in certain groups of nerve cells, as a kind of memory ', so that when they are roused into activity, the voluntary act will follow, as it probably did on the first occasion when the stimulus was applied. Finally, the irritating body may be seen, and the effects of the image formed optically on the retina are carried by the optic nerve to the corpora quadrigemina^ and from these to the visual centres in the cortex. Again, a memory of this impression may remain, and may be called into action by nervous influences coming from other parts of the brain, so that a vision of the irritat- ing body may afterwards arise into consciousness, so vividly as to call forth movements similar in character, although, probably, not so intense, as those which occurred in the first instance. This revivication of old impressions is most likely to occur when the upper centres are some- what in abeyance, as in the phenomena of hypnotism and somnambulism. Sensory impressions, however, are not only carried to the cerebral cortex, there awakening consciousness, but they are also conveyed, and many of them in the first instance, to the cerebellum, and in this organ they set in action the physiological mechanism that results in co- ordinated movements. It is not improbable that the sensory areas of the body have corresponding areas in the gray matter on the surface of the cerebellar convolutions. Thus the cerebellum is the organ that gives a rhythmic character to certain movements of the body, as those of walking, flying, swimming, etc., and probably it is only when these movements become associated with sensation, or are voluntary, that the centres in the cerebral cortex come into play. Again, if an external object acts at the same time on different organs of sense, as when we hold a rose in the Physiological Conditions of Sensation 295 hand, admire its colour, and enjoy its delicious perfume, the various sensations thus related to molecular movements in different parts of the cortex are combined by the action of the numerous fibres passing from centre to centre, and the result is a conscious perception of the thing as a whole. These fibres may be called fibres of association, because they combine impressions that have reached various sen- sory cortical centres. It is evident that such a combination of impressions may also give rise to various movements of the limb, or of the muscles of expression, and that the impressions will be more or less vivid as the exciting causes are strong or weak. If they are vivid, or, in other words, if the molecular changes in the nerve cells of parti- cular parts of the cortex of the brain are intense, they will have both a tendency to last after the exciting cause has been removed, and a tendency to be renewed by a slighter stimulus than was at first necessary to produce them. This is the physiological, or organic, foundation for memory, and also for the mental process known as the association oj ideas. Further, if such molecular processes, by frequent repetition, stamp a certain character on particular parts of the cerebral cortex, so as to be transmitted! According to the laws of heredity, then we have a physiological basis for innate tendencies or intuitions. The brain of one man differs from another in this respect. The greater the num- ber and variety of impressions made on an individual, the greater will be the number and variety of the molecular movements in the cells of the cortex, and the greater the number and variety of resulting mental and reflex pheno- mena. So intense may these processes be that they may be called into action by a stimulus from another part of the brain, as when irritation of the corpora quadrigemina by Indian hemp awakens in the cells of the visual centres of the cortex those changes which are associated in the mind 296 Physiology of the Senses with long-forgotten visual impressions, and the person sees passing before him a phantasmagoria of brilliantly-coloured images. These may also arise spontaneously, but the apparent spontaneity, however, is dependent on a stimulus so feeble as to escape notice, as when the sight of an object suddenly and almost unconsciously awakens memories of past events, and brings before the mind's eye forms and colours that long before produced impressions on the organs of sense. Many nervous phenomena are at first in a sense volun- tary, and by and by they become more and more of a reflex character, and are less and less associated with the higher consciousness. Thus a child acquires powers of walking by repeated efforts involving volition, judgment, and per- ception of different impressions, but the same movements of locomotion may be unconsciously performed by an adult. Familiar examples also are seen in the unconscious dexterity of movement of a skilful performer on a musical instrument, or in the deft movements of a cunning artificer. So is it even with psychical operations involving the action of the brain, and th^t>rain cortex may, as in unconscious cerebra- tion, pass thqfcRh molecular processes which result in the unconscious perrormance of actions that would be regarded as the result of mental processes, if the person were con- scious. Many instinctive actions are probably in this sense of an unconscious character. There can be no doubt that even in men the brain may work unconsciously, and the product may suddenly start out into consciousness. Facility of mental acquirement means a certain receptive- ness for particular kinds of molecular action. Other per- sonal factors come into operation, such as the power oj choice of particular impressions, the degree of attention paid to them at the time (depending largely on strength of will), the degree of stability of the results of the molecular move- Physiological Conditions of Sensation 297 ments that have been excited, and the power of associa- tion of different impressions. Each of these factors has a physiological basis peculiar to each individual. They are susceptible of being extended and strengthened by exercise, and just as muscular exercise causes an increased growth of muscular fibre, so regulated mental exercise must develop and strengthen the tissue of the brain. Thus one man differs from another in the primitive constitution of his nerve centres. This determines his degree of intelligence, power of accurate judgment, and aptitude for special kinds of work. These qualities are determined chiefly by inherit- ance from ancestors who have thus given their descendant a groundwork of mental character. In the next place, the influence of a man's environment develops to a greater or less extent this and that faculty. This is the rational basis of all educative processes. Again, the degree of excitability of the nerve centres varies considerably among individuals, and it also may be influenced by exercise. On this depends the aptitude for reflex acts of all kinds. Lastly, there may be a greater or less influence exerted by the higher over the lower centres, or, in other words, a great(ayDr less degree of inhibitory power. The power of theWp, which may also be strengthened by exercise, or weallned by yielding to disease, or by tame compliance, depends on this factor. Thus by a study of nervous actions, as connected with and stimulated by impressions on the organs of sense, we have constructed a physiological basif of character, and that with- out admitting the truth of an exclusively materialistic hypo- thesis. Behind all brain action, although closely connected with it, there is the strongest probability of the existence of an immaterial agent of which Spenser wrote in his Hymn in Honour of Beauty : " For of the soul the body form doth take, For soul is form, and doth the body make." APPENDIX I THE ACTION OF LIGHT ON THE RETINA AT p. 150 reference is made to the electrical change that occurs when light falls on the living retina. A full description of this remarkable phenomenon was out of place at that part of the book, but inasmuch as it is the only example we have of a known physical process occurring in a terminal organ of sense, it merits here a further notice. For the detection of electrical currents in living tissues a sensitive galvanometer of high resistance must be employed. The currents are led off the living tissues by electrodes that are so constructed as to be unpolarisable — that is to say, they do not them- selves generate any current, nor are they altered by the passage of even a feeble current through them, so as to give rise to any electrical action. They simply lead off to the galvanometer any current that may exist. Such electrodes are variously constructed ; but a con- venient form is a trough of zinc, resting on insulating plates of vulcanite, amalgamated on the inner surface, and filled with a saturated solution of sulphate of zinc. A pad of blotting-paper, wet with the sulphate of zinc solution, is placed into each trough, and on the pad a bit of clay, moistened with saliva, is laid, so as to pro- tect any animal tissues placed on the clay from the irritant action of the sulphate of zinc. The electrodes, so prepared, are connected with the galvanometer. A frog's eye is dissected out (after the animal has been decapitated, and all sensation has been lost), and is so placed on the pads of clay that one pad touches the middle of the surface of the cornea, and the other the posterior surface of the eye- ball and the transverse section of the optic nerve. A current, which we may call the "resting-eye current," is shown by a deflection of the needle of the galvanometer. It can be shown that this current 300 Physiology of the Senses passes from the corneal surface through the galvanometer and back to the posterior surface of the eyeball — that is to say, the eyeball acts like a little galvanic element, the positive pole of which is the cornea and the negative pole the transverse section of the optic nerve. The eye is now covered with a blackened box so as to keep it in the dark, and the box is provided with a shutter by which the light may be shut off or admitted at pleasure. When we open the shutter, and allow light to fall upon the eye, the needle of the gal- vanometer will be seen to swing in the direction that indicates an increase in the current. If light is allowed to act on the eye for a few minutes, the current diminishes, falls off in strength as the retina becomes fatigued, and soon becomes less than it was when light was allowed to fall on the eye. If the light is allowed to act sufficiently long, the current becomes less and less until it reaches zero. If, however, we remove the light by closing the shutter before the retina has become too fatigued, there is at once a second in- crease in the strength of the current again indicated by a swing of the galvanometer needle, then a rapid diminution, and soon the needle becomes almost stationary. These are the details of a single experiment ; and they show that light alters the electrical condition of the eye, the impact of light causing an increase, its continued action a diminution, and its removal another increase in the " resting- eye current." It can be shown that the effect is due to the action of light on the retina, because if -this structure be removed, light will produce no variation in any, current that may be got from other structures. The effect is due to light and not to heat, because it is easy to absorb the heat rays, and still allow the light to pass, and vice versd. In both cases it is only when light rays reach the retina that the effect is obtained. These variations have been seen in the eyes of inverte- brates and vertebrates, and even in the eye of man himself. Further, by allowing the different rays of the spectrum to fall on the eye, we can show that the luminous yellow rays produce more effect than the less luminous green, red, blue, or violet rays, and that the sum of the effects of the different rays is almost that of white light. It can also be demonstrated that the effects of varying intensities of light agree with the laws formulating the relation between the strength of the stimulus and the strength of the resulting sensation referred to on p. 39. The importance of this observation is due to the indica- tion it gives that the stimulus-sensation-ratio may be a function of the terminal organ as well as of the brain. Appendix I 301 The electrical variations above described may be physical indications of chemical phenomena known to occur in the retina. This, however, has not been proved. It is conceivable, as an alter- native hypothesis, that the rods and cones act as transforming struc- tures, changing the waves of light into electrical variations that pass along the fibres of the optic nerve. Electrical variations are the only phenomena that have yet been demonstrated in a nerve fibre during the passage along it of a nervous impulse ; and if, as the physicists assert, light waves are only short electrical waves, the hypothesis suggested is not unreasonable. These electrical changes in the retina, caused by the action of light, were independently discovered by Holmgren in Upsala, and by Dewar and M'Kendrick in Edinburgh, between 1870 and 1873. l 1 Dewar and M'Kendrick, Proceedings of Royal Society of Edin- burgh, 1874. Also M'Kendrick's Text -Book of Physiology, vol. ii. p. 627. APPENDIX II DERIVATIONS OF SCIENTIFIC TERMS ABERRATION, L. aJ), away ; erro, erratum, to wander Actinic, Gr. aktis, a sunbeam Acustica, Gr. akouo, to hear -SIsthesiometer, Gr. asthesis, feeling ; metron, a measure Afferent, L. ad, to ; fero, I carry Alkaloid, Arab, alkali ; Gr. eidos, likeness Allotropic, Gr. allotropos, of a different nature Ametropia, Gr. a, not ; metron, measure ; ops, the eye Amplitude, L. amplitude, largeness Ampulla, L. ampulla, a bottle Anaesthesia, Gr. -a, without ; asthesis, perception Analgesia, Gr. a, without ; algos, pain Anode, Gr. ana, up ; hodos, a way Anosmia, Gr. a, without ; osme, smell Aqueous, L. aqua, water Arborescent, L. arboresco, to become a tree Astigmatism, Gr. a, without ; stigma, a point Ataxia, Gr. a, without ; taxis, arrangement Auditory, L. audio, auditiim, to hear Aura, Gr. ao, to breathe Auricle, L. auriculus, dim. of auris, an ear Automatic, Gr. automates, of one's own accord BASSOON, Gr. basis, base ; a wind instrument giving a low note Biconvex, L. bis, twice ; con, together ; veho, vectum, to carry Binary, L. bina, a pair Appendix II 303 Binaural, L. bis, twice ; audio, I hear Binocular, L. bis, twice ; oculus, the eye CALLOSUM, L. callosus, thick-skinned Camera, L. camera, a chamber Capillary, L. capillus, a hair Cardinal, L. cardo, a hinge Cataract, Gr. kata, down ; arasso, to fall Cerebellum, L. cerebellum, dim. of cerebrum, the little brain Cerebrum, L. cerebrum, the brain Cerumen, L. cera, wax Choroid, Gr. chorion, skin ; ^zifoy, likeness Chromatic, Gr. chroma, colour Ciliary, L. cilium, an eyelash Cilium (//. cilia), L. cilium, an eyelash Circumvallate, L. circum, around ; vallum, a wall Cochlea, Gr. kochlias, a snail with a shell Coma, Gr. koma, drowsiness Commissure, L. «w*, together ; mitto, missum, to send Complementary, L. f0w, together ; pleo, to fill Congenital, L. congenitus, born together with Conjugate, L. w», together ; jugum, a yoke Conjunctiva, L. con, together ; jungo, junctum, to join Consciousness, L. c0», together ; scio, I know Convergence, L. #?», together ; vergo, to bend Convolution, L. convolve, convolution, to roll Corium, Gr. chorion, skin Cornea, L. cornu, a horn Corona, L. corona, a crown Corpus (//. corpora), L. corpus, a body Corpuscle, L. corpusculus, dim. of corpus, a body Cortex, L. cortex, bark Cranium, Gr. kranion, the skull Cribriform, L. cribrum, a sieve ; >rwa, likeness Crista, L. ;mto, a crest Cuneus, L. cuneus, a wedge Cupula, L. cupula, a small cup DALTONISM, Dalton, a celebrated chemist who was colour-blind Decussation, L. decusso, to place crosswise in the form of an X Dental, L. dens, dentis, a tooth 304 Physiology of the Senses Derma, Gr. derma, the skin Diabetes, Gr. dia, through ; baino, to go Diaphragm, Gr. dia, across ; phrasso, to fence Dioptrics, Gr. di, through ; horao, I see Dispersion, L. dis, asunder ; spargo, to scatter Dissonance, L. dis, asunder ; sonans, sounding Dynamical, Gr. dynamis, power EFFERENT, L. ex, out ; fcro, I carry Electrode, G r. elektron, amber ; hodos, a way Electrolysis, Gr. elektron, amber ; lysis, a softening Emmetropic, Gr. en, in ; metron, measure ; ops, the eyt Endolymph, Gr. endon, within ; lympha, water Entoptic, Gr. entos, within ; ops, the eye Epidermis, Gr. epi, upon ; derma, skin Epiglottis, Gr. epi, upon ; glotta, a tongue Erectile, L. e, out ; recto, to make straight Ether, Gr. aither, the upper air Ethmoid, Gr. ethmos, a sieve ; eidos, likeness FAUCES, 'L. fauces, the gullet Fenestra, L. fenestra, a window Fibril, L. fibra, a filament Filament, L. filum, a thread Filiform, L. filum, a thread ; forma, form Fluorescence, "L.Jiuo, I flow Focus, L. focus, £ fireplace Foliata, L. fotium, a leaf Follicle, L. follicuhis, dim. offollis, a wind ball or bag Foramina, L. foro, to bore Formication, L. formica, an ant Fornicatus, L. fornicatus, arched Fovea, L. fovea, a small pit Function, L. fungor, functum, to discharge an office Fundus, L. fundus, the bottom Fungiform, L. fungus, a mushroom ; forma, form Fuscin, L.fusats, tawny GALVANOMETER, Galvani, the discoverer of certain electrical pheno mena ; metron, a measure Ganglion (pi. ganglia), Gr. ganglion, a tumour under the skin Glosso-pharyngeal, Gr. glossa, the tongue ; pharynx, the throat Appendix II 305 Gustatory, L. gustatus, tasted Gyri, Gr. gyros, a circuit HEMORRHAGE, Gr. haima, blood ; rheo, to flow Hamulus, L. dim. of hamus, a hook Helicotrema, Gr. helix, a spiral ; trema, a perforation Hemiansesthesia. Gr. hemi, half ; a, without ; asthesis, feeling Heteronomous, Gr. heteros, another ; onoma, a name Hippocampus, Gr. hippos, a horse ; kampos, a sea-monster Homologous, Gr. homos, the same ; logos, a discourse Homonomous, Gr. homos, the same ; onoma, a name Horopter, Gr. horos, a boundary ; opter, a spectator Hyaloid, Gr. hyalos, glass ; eidos, a likeness Hypermetropia, Gr. hyper, beyond ; metron, measure ; ops, the eye Hypnotism, Gr. hypnos, sleep Hypoglossal, Gr. hypo, under ; glossa, the tongue Hypometropia, Gr. hypo, under ; metron, measure ; ops, the eye ILLUSION, L. in, in ; ludo, lusum, to play Incus, L. incus, an anvil Index, L. in, in ; dico, to proclaim Intermmcial, L. inter, between ; nuntius, a messenger Intuition, L. intus, within ; itum, to go Iris, Gr. iris, the rainbow JAUNDICE, Yr.jaune, yellow KATHODE, Gr. kata, down ; hodos, a way Klang, Ger. klang, the quality of a sound LABYRINTH, Gr. labyrinthos, a labyrinth Lachrymal, L. lachryma, a tear Lamella, L. lamella, dim. of lamina, a small plate Lamina, L. lamina, a small plate Laxator, "L. laxo, to loosen Lens, L. &#.y, a lentil Lenticular, L. dim. of /*«, a singer of sweet muyic TAPETUM, Gr. tapes, tapestry Telestereoscope, Gr. tele, at a distance ; stereos, solid ; skopeo, 1 see Temporo-sphenoidal, L. tempora, the temples ; Gr. sphen, a wedge ; «!o5«, likeness Thalamus, Gr. thalamos, a couch Thaumatrope, Gr. thauma, wonder ; tropos, a turning Timbre, Fr. timbre, the sound of a bell, the voice Translucent, L. trans, through ; luceo, to shine Triturate, L, tritus, rubbed Appendix II Turbinated, L. turbinatus, pointed Tympanum, Gr. tympanon, a drum UMBO, L. umbo, the boss of a shield Uncinate, L. uncus, a hook Undulatory, L. unda, a wave Utricle, L. dim. of uter, a leathern bag Uvula, L. dim. of uva, a grape VAS, L. vas, a vessel Vertebrate, L. verto, I turn Vestibule, L. vestibulum, a threshold Vibration, L. vibro, to quiver Vibrissae, L. vibro, to quiver Vidian, after Vidius, who described the Vidian nervr Viscera, L. viscera, the bowels Vitreous, L. vitrum, glass Volatility, L. volo, volatum, to fly Vorticosa, L. verto, to turn ZERO, Arab, tsaphara, empty Zonule, L. dim. of zona, a belt INDEX ABERRATION, spherical, 122 ; chromatic, 124 ; chromatic, of eye, 131 ; spherical, of eye, 132 ABNEY on colour vision, 170 Absolute sensitiveness, 56 Accommodation of eye for dis- tance, 135 Aerial perspective, 188 ^Esthesiometer, 55 After-image, 153 ; positive, 154 ; negative, 154 ; coloured, 161 After- tactile impressions, 58 AjUGARI, LUCREZIA, voice of, 246 Albinos, 101 Ametropic eye, 138 Ampulla, 224 Anaesthesia,, 16 Analgesia, 16 Analogy between touch and hear- ing, S3 Angle of convergence, 188 Anosmia, 94 Antennae of insects, 52 Apex-process, 28 Appendages of the skin, 43 Aqueous humour, 100 Area of distinct vision, 145 ARENSOHN on odours, 91, 92 ARISTOTLE'S experiment, 61 Aromatic bodies, 87 Association, fibres of, 295 ; of ideas. 295 Astigmatism, 132 Auditory hairs, 227 ; nerve, 223 ; teeth, 235 Aura of epilepsy, 33 Auricle, 200 ; its function, 201 Automatic movements, 20 BALFOUR, F. M., on sensory apparatus, 8 Beats, 259 Beat-tones, 260 BEAUNIS on odours, 92 Binaural audition, 283 Binocular vision, 170 ; visual field, 177 Birds, cochlea of, 271 Blindness, psychical, 31 Blind spot, 149 BOWMAN, glands of, in nose, 85 ; spiral ligament of, 229 ; ante- rior and posterior elastic lamina of, 99 Brightness of colour, 159 BROWN, A. CRUM, on semicircular canals, 268 BRUCH, membrane of, 102 Bulb, 1 8 CALLOSO-MARGINAL fissure, 27 Canalis reurnens, 225 Canals, semicircular, their de- velopment, 224; in equilibrium. 267 Cardinal points, 125 312 Physiology of the Senses Cataract, 107 Cells of cortex of brain, 29 Centre for hearing, 32 ; for per- ception of heat and cold, 35 ; of rotation of eyes, 171 ; for taste and smell, 34 ; for touch, 34; for vision in cortex cerebri, 30 Cerebellum, 19 Cerebral peduncles, 22 Cerebration, unconscious, 296 Cerebrum, 22 Cerumen, 204 Chain of bones, 209 ; movements of, 211 ; transmission of vibra- tions by, 218 Chamber, anterior, 100 ; pos- terior, 1 08 Choice, power of, 296 Chorda tympani, 205 Choroid, 99 Ciliary arteries and veins, 100 ; ganglion, in ; muscle, 102; nerves, in ; processes, the, 102 Circle of sensibility, 62 CLAUDIUS'S cells, 236 CLELAND, theory as to seat of consciousness, 288 Cochlea, 228 ; its function, 273 Cochlear canal, 225, 228, 230 Cold spots, 64 - Colour blindness, 159 Colour, sensation of, 155 Colour of the skin, 43 Colour vision theories, 161 Coma, 286 Common sensations, 35 Compasses for touch, 54 Cones of retina, 103, 104 Confusion colours, 160 Conjunctiva, 99 Consciousness, 286 ; seat of, 288 ; not a form of energy, 291 Contrast of colours, 161 Convolutions of brain, 24 Co-ordinated movements, 294 Corium, 41 Cornea, 98 Corona radiata, 28 Corpora quadrigemina, 22 Corpus callosum, 24 ; striatum, 23. 35 CORTI, organ of, 232 Cribriform plate of ethmoid bone, 83 Crista acustica, 227 Crystalline lens, 106, 107 Cuneus, 28 Cupula, 227 Cutis vera, 41 CYON on semicircular canals, 267 DALTONISM, 159 Deafness resulting from destruc- tion of cortical centre, 34 Decussation of nerve fibres, 15 Degeneration of nerve fibre, 13 DEITER'S cells, 235 ; their func- tion, 266 Delicacy of sense of smell, 92 Derma, 41, 42 DESCEMET, membrane of, 99 Dewar, observations on physio- logical action of light, 301 Dioptrics, laws of, 115 Dissonance, 260 Distance, estimation of, 187 Distinct vision, 175 DONDERS on the eye, 171 Drum, drum-head, 199, 202 Ductus endolymphaticus, 225 EAR, external, 200 ; middle, 204 ; internal, 223 ; their functions, 264, 265 Emmetropic eye, 138 End-bulbs, 47 Endolymph, 266 Entoptic phenomena, 141 Epidermic structures, their func- tions, 43, 51 Epidermis, 41, 42 Epithelium, olfactory, 84 Ethmoid bone, 81 EULENBERG, sensitiveness of skin, 57 Index 313 Eustachian tube, 82, 199, 207 Externality in sensation, 40 Eye, adjustment for different dis- tances, 134 ; examination of interior of, 143 ; dioptric sys- tem of, 127 ; its defects as an optical instrument, 131 Eyeball, structure of, 97 ; con- tents of, 105 FATIGUE of nerve, 4 FECHNER'S law of sensation, 39 Fenestra ovalis, 206 ; rotunda, 207, 229 Fenestrated membrane, 236 FERRIER on brain, 29, 34 FESTING on colour vision, 170 Fissure of ROLANDO, 26 ; of SYLVIUS, 26 Flavour, 74 Flowers and odours, 89 Fluorescence, 116 Focal points, 125, 128 Focus, principal and conjugate, 120 Follicle of hair, 50 Foramina nervina, 237 Form, judgment as to, 194 Formication, 35 FORSTER, GASPARD, voice of, 245 Fovea centralis, 105 FRITSCH on brain, 29 Frontal lobe of brain, 26 Fundamental colours, 158 ; tone, 250, 251, 253 Fuscin, 105 Fusion of tactile impressions, 58 G ALTON'S observations on the blind, 56 Ganglia, 9 ; spiral ganglion, 237 GASPARD FORSTER, basso, 245 GAUSS, cardinal points of, 125 GLASER, fissure of, 205 Glosso-pharyngeal nerve, 72, 73 GOLDSCHEIDER, hot and cold spots, 64 GRAHAM on odours, 93 GRANDRY'S corpuscles, 46 GRATIOLET, radiation of, 30, 32 Gustatory nerves, 72 Gymnema sylvestre, 77 Gyri operti, 26 Gyrus, a convolution of brain, 24 ; fornicatus, 27, 35 ; hippo- campi, 27 ; a centre for touch, 35 ; uncinatus, a centre for smell, 34 HAIR-CELLS, inner, 233 ; outer, 235 ; their function, 266 HALL, STANLEY, theory of colour vision, 164 Hallucinations, auditory, 279 Hamulus, 228 Harmonic motion, 248 Harmonics, 253 HARTMANN, Von, 287 Hearing, 198, centre in cerebrum for, 32 ; range of, 245 Hearing affected by drugs, 278 Helicotrema, 228 HELMHOLTZ, Von, theory of colour vision, 162, 169 ; ophthalmo- scope of, 143 ; telestereoscope of, 184 ; resonators of, 251 ; syren of, 242 ; on quality of sounds, 256, 257, 260 ; theory as to function of cochlea, 271 Hemianaesthesia, 35 HENRY, CH. , on odours, 86, 94 HENSEN'S spot, 235 ; cells, 236 ; on Mysis, 265 HERBST'S corpuscles, 50 HERING, theory of colour vision, 165 Heteronomous images, 179 HITZIG on brain, 29 HOLMGREN, observations on phy- siological action of light, 301 Homonomous images, 178 Horopter, 178 HORSLEY, areasofbrain, 29, 34,35 Hot spots, 64 Hue of colour, 159 Hyaloid membrane, 105 Physiology of the Senses Hypermetropic eye, 138 Hypometropic eye, 138 ILLUSIONS of vision, 192 Images formed by lenses, 120, 121 Incus, 210 Intensity, 246 ; of odours, 92 ; of sensation, 37, 38 ; of taste, 76 Internal capsule, 23 Intuitions, 295 Iris, the, 100 Iridescence of epidermic struc- tures, 45 Irradiation, 140, 154 Island of REIL, 26 JACOB'S membrane, 103 KLANG of musical tone, 247 KOENIG, analysis of compound tones, 254 KRAUSE'S end-bulbs, 47 ; theory as to touch, 63 LABYRINTH, membranous, 199, 225 ; osseous, 223 Lachrymal gland, 97 LADD on colour sense, 168 LAMBERT on colours, 156 Lamina cribrosa of the eye, 109 ; spiralis ossea, 228 ; mem- branacea, 230 LANGERHANS, cells of, 45 Laxator tympani, 205 LE CONTE, divergence of visual axes, 174 Lens, biconvex, 119 Lenticular process of incus, 210 LIEGEOIS on odours, 90 Light, physiological action of, 299; physical nature of, 115; reflection and refraction of, 116 Limbus, 230 Line of regard, 131 ; vision, 131 LISTING, cardinal points of, 125 ; schematic and reduced eye of, 130 Lobes of the brain, 26 Locomotor ataxia, 17 Loudness of sound, 246 Lower limit of excitation, 37 LUCREZIA AJUGARI, soprano, 246 Luminiferous ether, 97, 115 MACH, action of semicircular canals, 268 Macula acustica, 226 MAJENDIE, paths of sensory fibres, 10 Malleus, 209 MALPIGHI, stratum of, 42 Marginal gyms, 27 Massiveness of taste, 75 Manubrium of malleus, 209 M'KENDRICK, observations on physiological action of light, 301 Meatus, external auditory, 202 ; internal auditory, 225 Meatuses of nose, 81 Medulla oblongata, 18, 19 MEISSNER'S touch corpuscles, 47 Melanin, 105 Membrana basilaris, 230 ; tec- toria, 237 ; tympani, 199, 202, 205 ; response to sound-waves, 214 Memory, 294 ; of sounds, 282 MERKEL'S corpuscles, 46 Minimum visibile, 148 Modiolus, 225, 228 MCEBIUS on cardinal points, 125 Molar movement, 219, 221 Molecular movements, 219 Motion, perception of, 193 MUELLER, H., sensitive layer of retina, 143 MUELLER'S sustentacular fibres, 103 MUNK, sensory centres, 31, 34 Muscae volitantes, 141 Muscles of the eye, 172 Muscular sense, 36, 68 Index Musical tones, 240 Myopic eye, 138 Mysis, experiment on, 265, 272 NARES, anterior and posterior, 81 Nasal cartilages, 81 ; mucous membrane, 81 ; cavities, 80 Near point of vision, 137 Nerves, afferent and efferent, 10 ; their structure, n Nerve current, 5 ; rate of, 6 ; cells, their origin, 9 ; matter, 3 Nerves, fatigue of nerve, 4 ; of the nose, 82 ; of the tongue, 78 Nerve-endings in the tactile hairs, 50 ; free, 45 ; in corpuscles, 45 Neuro-epithelium, 8 Neuroglia, 28 NEWTON, analysis of light, 116 NILSSON, voice of, 245 Nodal points, 126, 129 Noises, 240, 262 Normal eye, average, 128 Nose, vestibular portion of, 83 ; respiratory, 83 ; olfactory, 84 ; Nose-leaves of bats, 52 OBLIQUE muscles, 172 Occipital lobe of brain, 26 Occipito - angular area, a visual centre, 31 ; blindness resulting from destruction of, 32 Odoroscope, 90 Odorous substances, their chemi- cal nature, 87 Odours, their influence on respira- tion, 94 ; and heat absorption, 89 ; pure and mixed, 93 ; and surface tension, 90 Olfactory cells, 85 ; epithelium, 84 ; lobes, 83 ; nerves, 83 Ophthalmoscope, its principle, Optic commissure, 109 ; lobes, 23 ; nerve, 109 ; papilla, 149 ; pore, 102 ; thalami, 23 ; tracts, 22, no Orbits, 170 Organ of CORTI, 231 Otoconia, 227 Otoliths, 227 Overtones, 253 OWEN on tactile hairs, 52 Ozone and odours, 90 PACINI'S corpuscles, 46, 48, 49 ; their function, 53 Pain, 67 ; its quality, 68 Papillae, 42 ; filiform, fungiform, circumvallate, 70 ; foliatae, 71 Parietal lobe of brain, 26 Partial tones, 253 Peduncles, cerebral, 22 Perception time, 6 Perilymph, 228 Perspective, aerial, 188 PETIT, canal of, 106 Phakoscope, 136 Phalangae, 236 Phase of vibration, affecting quality of tone, 257 Phosgenes, 152 Pigments, 157 Pitch of musical tones, 242 Pituita, 82 Points, remote and near, of vision, 137 Pons VAROLII, 21 Position, primary, secondary, tertiary, of eyeball, 171 Presbyopic eye, 140 PREVOST on odours, 90 Prickle cells in skin, 43 Principal points, 129 ; planes, I2S Prisms, 118 Processus cochleariformis, 207 ; gracilis of malleus, 210 Promontory, 207 Protecting cells of taste bud, 73 Physiology of the Senses Protoplasm, its chemical consti- tution, 3 ; its instability, 4 Pseudoscope, 184 Psychical blindness, 31 ; deafness, 34 Psycho-physical time, 6 Pupil, 100 ; movements of, 1 1 1 Purity of colour, 159 PURKINJE'S figures, 142 Purple of retina, 151 QUALITY of musical tones, 247, 256 ; of sensation, 36 Quantitative character of sensa- tion, 37 RAMSAY on odours, 87, 93 Ray, course of, in dioptric system, 126 Rectus muscle, 172 Reduced eye, 130 Reflection of rays of light, 116 Reflex mechanism, 289 Refraction of rays of light, 117 ; index of, 118 Registers of voice, 245 REIL, island of, 26 REISSNER'S membrane, 230 Resolving power of the eye, 147 Resonance, sympathetic, 255 Resonators, 251 ; analysis of tones by, 252 Retina, 103 ; appreciation of colour, 151 ; fundus of, 103 ; retinal impressions, 154; action of light on, 150 ; correspond- ing points of, 177 ; electric current of, 299 ; rods of, 103, 104 ; examination of, 143 Rod cells of taste bud, 72 Rods of CORTI, 232 RUTHERFORD, theory as to func- tion of cochlea, 270 SACCULE, 225 Scala tympani, 229 ; vestibuli, 228 SCHAEFER on sensory centres, 34, 35 SCHEINER'S experiment, 137 Schematic eye, 130 SCHLEMM, canal of, 101 Schneiderian membrane, 81 SCHULTZE on odours, 93 SCHWANN, white substance of, ii ; primitive sheath of, n Sclerotic, 98 Semicircular canals, their forma- tion, 224 ; their function, 267 Sensorium, i Sense of equilibrium, 270 ; of hearing, 198 ; of innervation, 69 ; of locality, 56 ; of sight, 96 ; of smell, 80 ; of smell, its delicacy, 92 ; of taste, 70 ; of temperature, 64 ; of touch, Sensibility of the tongue, 78 Sensitiveness, absolute, 56 ; of the skin, 54 Sensory paths in spinal cord, 13 ; impressions, objectivity of, 40 ; time in, 6 ; mechanism of, i SHORE on tastes, 76 SIEVEKING, 55 Size of the retinal image, 148 ; estimation, 190 Smell, cerebral centres for, 34 ; physical cause of, 86 ; physio- logy of, 91 ; sense of, 80 Skin as excretory organ, 43 ; structure of, 41 ; true skin, 42 ; sensitiveness of, 54 Solidity, perception of, 180 Somnambulism, 196 Sound, 198 ; its externality, 277 ; its direction, 280 ; its distance, 281 ; its velocity, 220 Specific light of the retina, 152 Spectrum, solar, 116 SPENSER, relation of soul and body, 297 Index Spinal coid, 13 Spiral ganglion, 237 ; ligament, 229, 236 Stapes, 211 Stapedius muscle, 207 Stereoscope, 181 Stimulus and sensation, 36 Stirrup-bone, 211 Stratum corneum, of the skin, 42 ; lucidum of the skin, 43 ; mucosum of skin, 42 Structure of the skin , 41 ; of cortex cerebri, 29 Subjective sensations of odour, 94 ; tastes, 78 SUELZER on taste, 75 Sulci of the brain, 24 Sulcus spiralis, 230 Supporting cells of olfactory epi- thelium, 85 Suspensory ligament of lens, 106 Syren, 242 TACTILE cells, simple, 46 ; com- pound, 47 ; field, 60 ; hairs, 51 ; impressions, information from, 59 ; organs, their struc- ture, 45 TAIT, simple harmonic motion, 248 Tapetum, the, 101 TARTINI on overtones, 276 Taste, physical causes of, 73 ; solubility a condition of, 73 ; physiological conditions of, 74 ; classification of, 74 ; excitants of, 75 ; differentiation of, 76 ; massiveness of, 75 ; intensity of, 75 Taste buds or goblets, 71 Taste pore, 72 Telestereoscope, 184 Temperature, sense of, 64 Temporo - occipital convolution, 28 Temporo-sphenoidal lobe of brain, 26 Tensor tympani, 207 ; its func- tion, 265 Terminal organs, 2 Test colours, 160 Thalami optici, 23 Thaumatrope, 154 THOMSON (Lord Kelvin), simple harmonic motion, 248 Threshold of sensation, 37 Timbre of musical tone, 247 Tone, 240 Tongue, 70 Touch, sense of, 41 ; corpuscles, simple, 46 ; compound, 47 ; mechanism of, 52 ; theories as to, 62 Transmission of sound by cra- nium, 222 ; laws of, 281 Tuning-fork, 248 Tunnel of CORTI'S organ, 232 Turbinated bones of nose, 81 Tympanic groove, 205 Tympanum, 199, 204 TYNDALL on odours, 89 UMBO of tympanic membrane, 206 Uncinate gyrus, 27 Unconscious cerebration, 296 Utricle, 224 VALSALVA, experiments of, 208 Vas spirale, 230 VATER'S corpuscles, 48 Venas vorticosae, 100 VENTURI on odours, 90 Vestibule, 223 Vibrations of strings, 215 Vibrissae, tactile hairs, 51, 8 1 Visual angle, 145, 190; field, I76 Vitreous humour, 106 VOLKMANN, variation of acuteness of vision, 150 WAGNER'S touch corpuscles, 47 Wave-length, 220 WEBER on sensitiveness of the Physiology of the Senses skin, 54, 55 ; theory as to touch, 62 ; on odours, 91 Wheel oflife, 154 WHEWELL on astigmatism, 133 YELLOW spot, 102 YOUNG, THOMAS, theory of colour vision, 162 ; undulatory theory of light, 115 ZOELLNER'S lines, 192 Zone of oval nuclei, 85 ; 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